feat: add default IDE setting and multi-editor support with icons

Add comprehensive editor detection and selection system that allows users
to configure their preferred IDE for opening branches and worktrees.

## Server-side Changes

- Add `/api/worktree/available-editors` endpoint to detect installed editors
- Support detection via CLI commands (cursor, code, zed, subl, etc.)
- Support detection via macOS app bundles in /Applications and ~/Applications
- Detect editors: Cursor, VS Code, Zed, Sublime Text, Windsurf, Trae,
  Rider, WebStorm, Xcode, Android Studio, Antigravity, and file managers

## UI Changes

### Editor Icons
- Add new `editor-icons.tsx` with SVG icons for all supported editors
- Icons: Cursor, VS Code, Zed, Sublime Text, Windsurf, Trae, Rider,
  WebStorm, Xcode, Android Studio, Antigravity, Finder
- `getEditorIcon()` helper maps editor commands to appropriate icons

### Default IDE Setting
- Add "Default IDE" selector in Settings > Account section
- Options: Auto-detect (Cursor > VS Code > first available) or explicit choice
- Setting persists via `defaultEditorCommand` in global settings

### Worktree Dropdown Improvements
- Implement split-button UX for "Open In" action
- Click main area: opens directly in default IDE (single click)
- Click chevron: shows submenu with other editors + Copy Path
- Each editor shows with its branded icon

## Type & Store Changes

- Add `defaultEditorCommand: string | null` to GlobalSettings
- Add to app-store with `setDefaultEditorCommand` action
- Add to SETTINGS_FIELDS_TO_SYNC for persistence
- Add `useAvailableEditors` hook for fetching detected editors

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/ui/src/lib/electron.ts

@@ -1625,13 +1625,14 @@ function createMockWorktreeAPI(): WorktreeAPI {
       };
     },
 
-    openInEditor: async (worktreePath: string) => {
-      console.log('[Mock] Opening in editor:', worktreePath);
+    openInEditor: async (worktreePath: string, editorCommand?: string) => {
+      const editorName = editorCommand === 'cursor' ? 'Cursor' : 'VS Code';
+      console.log('[Mock] Opening in editor:', worktreePath, 'using:', editorName);
       return {
         success: true,
         result: {
-          message: `Opened ${worktreePath} in VS Code`,
-          editorName: 'VS Code',
+          message: `Opened ${worktreePath} in ${editorName}`,
+          editorName,
         },
       };
     },
@@ -1647,6 +1648,19 @@ function createMockWorktreeAPI(): WorktreeAPI {
       };
     },
 
+    getAvailableEditors: async () => {
+      console.log('[Mock] Getting available editors');
+      return {
+        success: true,
+        result: {
+          editors: [
+            { name: 'VS Code', command: 'code' },
+            { name: 'Finder', command: 'open' },
+          ],
+        },
+      };
+    },
+
     initGit: async (projectPath: string) => {
       console.log('[Mock] Initializing git:', projectPath);
       return {