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/server/src/routes/worktree/index.ts

@@ -24,6 +24,7 @@ import { createSwitchBranchHandler } from './routes/switch-branch.js';
 import {
   createOpenInEditorHandler,
   createGetDefaultEditorHandler,
+  createGetAvailableEditorsHandler,
 } from './routes/open-in-editor.js';
 import { createInitGitHandler } from './routes/init-git.js';
 import { createMigrateHandler } from './routes/migrate.js';
@@ -77,6 +78,7 @@ export function createWorktreeRoutes(): Router {
   router.post('/switch-branch', requireValidWorktree, createSwitchBranchHandler());
   router.post('/open-in-editor', validatePathParams('worktreePath'), createOpenInEditorHandler());
   router.get('/default-editor', createGetDefaultEditorHandler());
+  router.get('/available-editors', createGetAvailableEditorsHandler());
   router.post('/init-git', validatePathParams('projectPath'), createInitGitHandler());
   router.post('/migrate', createMigrateHandler());
   router.post(