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>

libs/types/src/settings.ts

@@ -565,6 +565,10 @@ export interface GlobalSettings {
   /** List of configured MCP servers for agent use */
   mcpServers: MCPServerConfig[];
 
+  // Editor Configuration
+  /** Default editor command for "Open In" action (null = auto-detect: Cursor > VS Code > first available) */
+  defaultEditorCommand: string | null;
+
   // Prompt Customization
   /** Custom prompts for Auto Mode, Agent Runner, Backlog Planning, and Enhancements */
   promptCustomization?: PromptCustomization;
@@ -819,6 +823,7 @@ export const DEFAULT_GLOBAL_SETTINGS: GlobalSettings = {
   codexAdditionalDirs: DEFAULT_CODEX_ADDITIONAL_DIRS,
   codexThreadId: undefined,
   mcpServers: [],
+  defaultEditorCommand: null,
   enableSkills: true,
   skillsSources: ['user', 'project'],
   enableSubagents: true,