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/ExplorerPanel.svelte

@@ -12,6 +12,7 @@
     FileImage,
     FileJson,
     FileSearch,
+    GitCompare,
     FileSpreadsheet,
     FileText,
     FileType,
@@ -33,11 +34,16 @@
     selectedExplorerKind: ExplorerNodeKind;
     hasRepository: boolean;
     isBusy: boolean;
+    language?: "en" | "de";
+    editorName?: string;
+    diffName?: string;
     onToggleFolder: (node: ExplorerNode) => void;
     onExpandAllFolders: () => void;
     onCollapseAllFolders: () => void;
     onSelectNode: (node: ExplorerNode) => void;
     onOpenFile: (node: ExplorerNode) => void;
+    onOpenInEditor: (node: ExplorerNode) => void;
+    onExternalDiff: (node: ExplorerNode) => void;
     onFileHistory: (node: ExplorerNode) => void;
     onBlame: (node: ExplorerNode) => void;
     collapsed?: boolean;
@@ -51,11 +57,16 @@
     selectedExplorerKind = "file",
     hasRepository = false,
     isBusy = false,
+    language = "en",
+    editorName = "Editor",
+    diffName = "diff tool",
     onToggleFolder = () => {},
     onExpandAllFolders = () => {},
     onCollapseAllFolders = () => {},
     onSelectNode = () => {},
     onOpenFile = () => {},
+    onOpenInEditor = () => {},
+    onExternalDiff = () => {},
     onFileHistory = () => {},
     onBlame = () => {},
     collapsed = false,
@@ -65,6 +76,7 @@
   let contextNode = $state<ExplorerNode | null>(null);
   let contextMenuX = $state(0);
   let contextMenuY = $state(0);
+  const isGerman = $derived(language === "de");
 
   function mergeExplorerStatus(current: FileStatusKind | null, next: FileStatusKind | null): FileStatusKind | null {
     if (!next) return current;
@@ -178,7 +190,7 @@
 
     contextNode = node;
     contextMenuX = Math.max(8, Math.min(event.clientX + 2, window.innerWidth - 192));
-    contextMenuY = Math.max(8, Math.min(event.clientY + 2, window.innerHeight - 132));
+    contextMenuY = Math.max(8, Math.min(event.clientY + 2, window.innerHeight - 220));
   }
 
   function closeFileContextMenu() {
@@ -192,6 +204,20 @@
     onOpenFile(node);
   }
 
+  function openContextFileInEditor() {
+    const node = contextNode;
+    if (!node || node.kind !== "file") return;
+    closeFileContextMenu();
+    onOpenInEditor(node);
+  }
+
+  function openContextExternalDiff() {
+    const node = contextNode;
+    if (!node || node.kind !== "file") return;
+    closeFileContextMenu();
+    onExternalDiff(node);
+  }
+
   function openContextBlame() {
     const node = contextNode;
     if (!node || node.kind !== "file") return;
@@ -213,6 +239,11 @@
   let explorerTree = $derived(buildExplorerTree(repoFiles));
   let visibleNodes = $derived(flattenExplorerTree(explorerTree, expandedExplorerPaths));
   let hasFolders = $derived(explorerTree.some((node) => node.kind === "folder"));
+  let selectedFileNode = $derived(
+    selectedExplorerKind === "file"
+      ? visibleNodes.find((node) => node.kind === "file" && node.path === selectedExplorerPath) ?? null
+      : null,
+  );
 </script>
 
 <svelte:window on:click={closeFileContextMenu} on:keydown={handleWindowKeydown} on:contextmenu|capture={closeFileContextMenu} />
@@ -224,6 +255,35 @@
       <h2 class="mt-0.5 text-ink text-base font-bold leading-tight">Files</h2>
     </div>
     <div class="explorer-head-actions">
+      <button
+        class="explorer-bulk-button explorer-tool-action"
+        type="button"
+        onclick={() => selectedFileNode && onOpenInEditor(selectedFileNode)}
+        disabled={isBusy || !selectedFileNode || selectedFileNode.status === "deleted"}
+        title={isGerman
+          ? `Ausgewählte Datei in ${editorName} öffnen`
+          : `Open selected file in ${editorName}`}
+        aria-label={isGerman
+          ? `Ausgewählte Datei in ${editorName} öffnen`
+          : `Open selected file in ${editorName}`}
+      >
+        <FileCode size={14} aria-hidden="true" />
+      </button>
+      <button
+        class="explorer-bulk-button explorer-tool-action"
+        type="button"
+        onclick={() => selectedFileNode && onExternalDiff(selectedFileNode)}
+        disabled={isBusy || !selectedFileNode || !selectedFileNode.tracked || selectedFileNode.status === "deleted"}
+        title={isGerman
+          ? `Ausgewählte Datei mit HEAD in ${diffName} vergleichen`
+          : `Compare selected file with HEAD in ${diffName}`}
+        aria-label={isGerman
+          ? `Ausgewählte Datei mit HEAD in ${diffName} vergleichen`
+          : `Compare selected file with HEAD in ${diffName}`}
+      >
+        <GitCompare size={14} aria-hidden="true" />
+      </button>
+      <span class="explorer-action-divider" aria-hidden="true"></span>
       <button
         class="explorer-bulk-button"
         type="button"
@@ -366,6 +426,14 @@
     tabindex="-1"
     aria-label={`Actions for ${contextNode.path}`}
   >
+    <button type="button" role="menuitem" onclick={openContextFileInEditor} disabled={contextNode.status === "deleted"}>
+      <FileCode size={14} aria-hidden="true" />
+      {isGerman ? `In ${editorName} öffnen` : `Open in ${editorName}`}
+    </button>
+    <button type="button" role="menuitem" onclick={openContextExternalDiff} disabled={!contextNode.tracked || contextNode.status === "deleted"}>
+      <GitCompare size={14} aria-hidden="true" />
+      {isGerman ? `Mit HEAD in ${diffName} vergleichen` : `Compare with HEAD in ${diffName}`}
+    </button>
     <button
       type="button"
       role="menuitem"