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}