feat(branches): add Merge Branch dialog and confirm flow

Introduce a modal dialog to configure and confirm branch merges. It
replaces the old prompt-based workflow and lets users choose a merge
strategy with localized labels. Confirming the dialog runs the merge,
refreshes views, and clears the pending merge target.

- Add a merge dialog UI with strategy options and keyboard support.
- Replace prompt-based merge flow with a state-driven confirm dialog.
- Include CSS for dialog layout, responsive sizing, and styling.

src/App.svelte

@@ -27,6 +27,7 @@
   import ExplorerPanel from "./lib/components/ExplorerPanel.svelte";
   import HistoryPanel from "./lib/components/HistoryPanel.svelte";
   import InitRepositoryDialog from "./lib/components/InitRepositoryDialog.svelte";
+  import MergeBranchDialog from "./lib/components/MergeBranchDialog.svelte";
   import NewBranchDialog from "./lib/components/NewBranchDialog.svelte";
   import RenameBranchDialog from "./lib/components/RenameBranchDialog.svelte";
   import ReflogDialog from "./lib/components/ReflogDialog.svelte";
@@ -169,6 +170,7 @@
     GitFileStatus,
     GitIgnoreKind,
     GitLfsStatus,
+    MergeStrategy,
     GitRepositoryFile,
     GitRemote,
     PullStrategy,
@@ -391,6 +393,7 @@
   let comparisonFromLabel = "";
   let comparisonToLabel = "";
   let newBranchCommit: GitCommit | null = null;
+  let mergeBranchTarget: GitBranchInfo | null = null;
   let renameBranchTarget: GitBranchInfo | null = null;
   let deleteBranchTarget: GitBranchInfo | null = null;
   let deleteBranchForce = false;
@@ -3207,11 +3210,15 @@
 
   async function merge(branch: GitBranchInfo) {
     if (!activeRepoPath || branch.current) return;
-    const strategy = (window.prompt("Merge strategy: default, squash, ff-only, or no-ff", "default") ?? "").trim();
-    if (!strategy) return;
-    if (!["default", "squash", "ff-only", "no-ff"].includes(strategy)) { errorMessage = "Unknown merge strategy."; return; }
+    mergeBranchTarget = branch;
+  }
+
+  async function confirmMerge(strategy: MergeStrategy) {
+    const branch = mergeBranchTarget;
+    if (!activeRepoPath || !branch || branch.current) return;
     await runOperation(`Merging ${branch.name}`, async () => {
-      applyStatus(await mergeBranch(activeRepoPath, branch.name, strategy as import("./lib/types").MergeStrategy));
+      applyStatus(await mergeBranch(activeRepoPath, branch.name, strategy));
+      mergeBranchTarget = null;
       await refreshRepositoryViews(activeRepoPath);
       trackEvent("branch_merged", {
         remote: branch.remote ? 1 : 0,
@@ -5926,6 +5933,17 @@
   />
 {/if}
 
+{#if mergeBranchTarget}
+  <MergeBranchDialog
+    branch={mergeBranchTarget}
+    currentBranch={status?.current_branch ?? ""}
+    {isBusy}
+    language={appLanguage}
+    onMerge={confirmMerge}
+    onClose={() => { if (!isBusy) mergeBranchTarget = null; }}
+  />
+{/if}
+
 {#if commitNoteTarget}
   <CommitNoteDialog
     commit={commitNoteTarget}

src/app.css

@@ -3885,6 +3885,13 @@
     max-height: calc(100vh - 32px);
     overflow: auto;
   }
+  .merge-branch-dialog {
+    display: block;
+    width: min(590px, calc(100vw - 32px));
+    height: auto;
+    max-height: calc(100vh - 32px);
+    overflow: auto;
+  }
   .rename-branch-dialog {
     display: block;
     width: min(520px, calc(100vw - 32px));
@@ -4207,6 +4214,41 @@
   .init-repository-error { color: var(--color-danger, #ff6b78) !important; }
   .init-repository-actions { padding-top: 2px; }
   .init-repository-actions > button { font-size: 11.5px; }
+  .merge-branch-header { padding: 15px 17px; }
+  .merge-branch-heading { display: flex; align-items: center; gap: 11px; }
+  .merge-branch-heading h2 { margin: 2px 0 0; color: var(--color-ink); font-size: 15px; line-height: 1.25; }
+  .merge-branch-icon {
+    display: grid;
+    place-items: center;
+    width: 36px;
+    height: 36px;
+    flex: 0 0 auto;
+    border: 1px solid color-mix(in srgb, var(--color-accent) 34%, var(--color-border));
+    border-radius: 9px;
+    color: var(--color-accent);
+    background: color-mix(in srgb, var(--color-accent) 10%, var(--color-surface-raised));
+  }
+  .merge-branch-form { display: flex; flex-direction: column; gap: 15px; padding: 17px; }
+  .merge-branch-route { display: grid; grid-template-columns: minmax(0, 1fr) auto minmax(0, 1fr); align-items: center; gap: 12px; padding: 11px 12px; border: 1px solid var(--color-border-subtle); border-radius: 9px; background: var(--color-surface-raised); }
+  .merge-branch-route > div { display: grid; gap: 3px; min-width: 0; }
+  .merge-branch-route > div:last-child { text-align: right; }
+  .merge-branch-route span { color: var(--color-ink-faint); font-size: 9px; font-weight: 800; letter-spacing: .05em; text-transform: uppercase; }
+  .merge-branch-route strong { overflow: hidden; color: var(--color-ink); font-family: var(--font-mono); font-size: 11.5px; text-overflow: ellipsis; white-space: nowrap; }
+  .merge-branch-route > svg { color: var(--color-accent); }
+  .merge-strategy-fieldset { min-width: 0; margin: 0; padding: 0; border: 0; }
+  .merge-strategy-fieldset legend { margin-bottom: 7px; color: var(--color-ink-faint); font-size: 9.5px; font-weight: 800; letter-spacing: .05em; text-transform: uppercase; }
+  .merge-strategy-grid { display: grid; grid-template-columns: repeat(2, minmax(0, 1fr)); gap: 8px; }
+  .merge-strategy-option { display: grid; grid-template-columns: auto minmax(0, 1fr); align-items: start; gap: 9px; min-width: 0; padding: 11px; border: 1px solid var(--color-border-subtle); border-radius: 8px; background: var(--color-surface-dim); cursor: pointer; }
+  .merge-strategy-option:hover { border-color: var(--color-border); background: var(--color-surface-hover); }
+  .merge-strategy-option.active { border-color: color-mix(in srgb, var(--color-accent) 52%, var(--color-border)); background: color-mix(in srgb, var(--color-accent) 8%, var(--color-surface-raised)); box-shadow: inset 2px 0 0 var(--color-accent); }
+  .merge-strategy-option > input { position: absolute; width: 1px; height: 1px; opacity: 0; pointer-events: none; }
+  .merge-strategy-check { display: grid; place-items: center; width: 17px; height: 17px; margin-top: 1px; border: 1px solid var(--color-border-input); border-radius: 50%; color: #fff; background: var(--color-surface-raised); }
+  .merge-strategy-option.active .merge-strategy-check { border-color: var(--color-accent); background: var(--color-accent); }
+  .merge-strategy-copy { display: grid; gap: 3px; min-width: 0; }
+  .merge-strategy-copy strong { color: var(--color-ink); font-size: 11.5px; }
+  .merge-strategy-copy small { color: var(--color-ink-faint); font-size: 9.5px; line-height: 1.4; }
+  .merge-branch-actions { display: flex; justify-content: flex-end; gap: 8px; padding-top: 1px; }
+  .merge-branch-actions > button { font-size: 11.5px; }
   .rename-branch-form {
     display: flex;
     flex-direction: column;

src/lib/components/MergeBranchDialog.svelte

@@ -0,0 +1,119 @@
+<script lang="ts">
+  import { onMount } from "svelte";
+  import { Check, GitMerge, LoaderCircle, X } from "@lucide/svelte";
+  import type { AppLanguage, GitBranch, MergeStrategy } from "../types";
+
+  interface Props {
+    branch: GitBranch;
+    currentBranch: string;
+    isBusy: boolean;
+    language: AppLanguage;
+    onMerge: (strategy: MergeStrategy) => void;
+    onClose: () => void;
+  }
+
+  let {
+    branch,
+    currentBranch,
+    isBusy = false,
+    language = "en",
+    onMerge = () => {},
+    onClose = () => {},
+  }: Props = $props();
+
+  let strategy = $state<MergeStrategy>("default");
+  const isGerman = $derived(language === "de");
+  const options = $derived([
+    {
+      value: "default" as const,
+      label: isGerman ? "Standard" : "Default",
+      description: isGerman ? "Git wählt Fast-forward oder erstellt einen Merge-Commit." : "Git chooses fast-forward or creates a merge commit.",
+    },
+    {
+      value: "squash" as const,
+      label: "Squash",
+      description: isGerman ? "Fasst alle Änderungen zu einem neuen Commit zusammen." : "Combines all changes into one new commit.",
+    },
+    {
+      value: "ff-only" as const,
+      label: "Fast-forward only",
+      description: isGerman ? "Bricht ab, wenn ein Merge-Commit erforderlich wäre." : "Stops if a merge commit would be required.",
+    },
+    {
+      value: "no-ff" as const,
+      label: "No fast-forward",
+      description: isGerman ? "Erstellt immer einen eigenen Merge-Commit." : "Always creates a dedicated merge commit.",
+    },
+  ]);
+
+  function submit(event: SubmitEvent) {
+    event.preventDefault();
+    if (!isBusy) onMerge(strategy);
+  }
+
+  function handleKeydown(event: KeyboardEvent) {
+    if (event.key === "Escape" && !isBusy) onClose();
+  }
+
+  onMount(() => {
+    window.addEventListener("keydown", handleKeydown);
+    return () => window.removeEventListener("keydown", handleKeydown);
+  });
+</script>
+
+<div class="dialog-backdrop app-chrome-backdrop" role="presentation">
+  <div class="dialog merge-branch-dialog" role="dialog" aria-modal="true" aria-labelledby="merge-branch-title" tabindex="-1">
+    <header class="dialog-header merge-branch-header">
+      <div class="merge-branch-heading">
+        <span class="merge-branch-icon" aria-hidden="true"><GitMerge size={18} /></span>
+        <div>
+          <span class="eyebrow">{isGerman ? "Branches zusammenführen" : "Combine branches"}</span>
+          <h2 id="merge-branch-title">{isGerman ? "Merge konfigurieren" : "Configure merge"}</h2>
+        </div>
+      </div>
+      <button class="dialog-close" type="button" onclick={onClose} disabled={isBusy} title={isGerman ? "Schließen" : "Close"}>
+        <X size={18} aria-hidden="true" />
+      </button>
+    </header>
+
+    <form class="merge-branch-form" onsubmit={submit}>
+      <div class="merge-branch-route" aria-label={isGerman ? "Merge-Richtung" : "Merge direction"}>
+        <div>
+          <span>{isGerman ? "Quell-Branch" : "Source branch"}</span>
+          <strong>{branch.name}</strong>
+        </div>
+        <GitMerge size={18} aria-hidden="true" />
+        <div>
+          <span>{isGerman ? "In aktuellen Branch" : "Into current branch"}</span>
+          <strong>{currentBranch || "HEAD"}</strong>
+        </div>
+      </div>
+
+      <fieldset class="merge-strategy-fieldset" disabled={isBusy}>
+        <legend>{isGerman ? "Merge-Strategie" : "Merge strategy"}</legend>
+        <div class="merge-strategy-grid">
+          {#each options as option}
+            <label class:active={strategy === option.value} class="merge-strategy-option">
+              <input type="radio" name="merge-strategy" value={option.value} bind:group={strategy} />
+              <span class="merge-strategy-check" aria-hidden="true">
+                {#if strategy === option.value}<Check size={13} />{/if}
+              </span>
+              <span class="merge-strategy-copy">
+                <strong>{option.label}</strong>
+                <small>{option.description}</small>
+              </span>
+            </label>
+          {/each}
+        </div>
+      </fieldset>
+
+      <div class="merge-branch-actions">
+        <button class="btn-secondary" type="button" onclick={onClose} disabled={isBusy}>{isGerman ? "Abbrechen" : "Cancel"}</button>
+        <button class="btn-primary" type="submit" disabled={isBusy}>
+          {#if isBusy}<LoaderCircle class="spin" size={16} aria-hidden="true" />{:else}<GitMerge size={16} aria-hidden="true" />{/if}
+          {isGerman ? "Branch mergen" : "Merge branch"}
+        </button>
+      </div>
+    </form>
+  </div>
+</div>