feat(external-tools): add cross-platform external tool discovery

Adds a new external tools subsystem to detect and launch
diff and editor tools across Windows, macOS, and Linux.
It exposes data models for tools, commands, and results to the UI
and serializes them for consumption by the app.

- Implement cross-platform discovery of editors and diff tools
- Expose serialized results to the UI for user selection
- Centralize per-OS known tool lists and overrides

src/lib/components/ResolveDialog.svelte

@@ -1,6 +1,6 @@
 <script lang="ts">
   import { untrack } from "svelte";
-  import { AlertCircle, Check, LoaderCircle, X } from "@lucide/svelte";
+  import { AlertCircle, Check, ExternalLink, GitMerge, LoaderCircle, X } from "@lucide/svelte";
   import type { ConflictChoice, ConflictFile, ConflictPart, GitFileStatus, PreparedResolution } from "../types";
 
   type ConflictRegion = Extract<ConflictPart, { kind: "conflict" }>;
@@ -24,6 +24,9 @@
     onSelectFile: (path: string) => void;
     onMarkResolved: (path: string, resolution: PreparedResolution) => void;
     onApply: () => void;
+    onExternalMerge: (path: string) => void | Promise<void>;
+    language?: "en" | "de";
+    mergeName?: string;
   }
 
   let {
@@ -37,7 +40,11 @@
     onSelectFile = () => {},
     onMarkResolved = () => {},
     onApply = () => {},
+    onExternalMerge = () => {},
+    language = "en",
+    mergeName = "merge tool",
   }: Props = $props();
+  const isGerman = $derived(language === "de");
 
   let conflictParts = $derived<ConflictPart[]>(
     conflict && !conflict.binary ? parseConflicts(conflict.content) : [],
@@ -268,9 +275,22 @@
         <span class="eyebrow">Resolve</span>
         <h2 class="dialog-title">Conflicts</h2>
       </div>
-      <button class="dialog-close" type="button" onclick={onClose} title="Close">
-        <X size={18} aria-hidden="true" />
-      </button>
+      <div class="dialog-header-actions">
+        {#if conflictTarget}
+          <div class="tool-surface-choice" aria-label={isGerman ? "Konflikt bearbeiten mit" : "Edit conflict with"}>
+            <span>{isGerman ? "Bearbeiten mit" : "Edit with"}</span>
+            <button class="active" type="button" aria-pressed="true" title={isGerman ? "In Gitty bearbeiten" : "Edit in Gitty"}>
+              <GitMerge size={13} aria-hidden="true" />Gitty
+            </button>
+            <button type="button" onclick={() => onExternalMerge(conflictTarget)} disabled={isBusy} title={isGerman ? `In ${mergeName} öffnen` : `Open in ${mergeName}`}>
+              <ExternalLink size={13} aria-hidden="true" /><span>{mergeName}</span>
+            </button>
+          </div>
+        {/if}
+        <button class="dialog-close" type="button" onclick={onClose} title={isGerman ? "Schließen" : "Close"} aria-label={isGerman ? "Konflikte schließen" : "Close conflicts"}>
+          <X size={18} aria-hidden="true" />
+        </button>
+      </div>
     </header>
 
     {#if conflictedFiles.length === 0}