feat: add external terminal support with cross-platform detection

Add support for opening worktree directories in external terminals
(iTerm2, Warp, Ghostty, System Terminal, etc.) while retaining the
integrated terminal as the default option.

Changes:
- Add terminal detection for macOS, Windows, and Linux
- Add "Open in Terminal" split-button in worktree dropdown
- Add external terminal selection in Settings > Terminal
- Add default open mode setting (new tab vs split)
- Display branch name in terminal panel header
- Support 20+ terminals across platforms

Part of #558, Closes #550

apps/ui/src/lib/http-api-client.ts

@@ -1808,6 +1808,11 @@ export class HttpApiClient implements ElectronAPI {
     getDefaultEditor: () => this.get('/api/worktree/default-editor'),
     getAvailableEditors: () => this.get('/api/worktree/available-editors'),
     refreshEditors: () => this.post('/api/worktree/refresh-editors', {}),
+    getAvailableTerminals: () => this.get('/api/worktree/available-terminals'),
+    getDefaultTerminal: () => this.get('/api/worktree/default-terminal'),
+    refreshTerminals: () => this.post('/api/worktree/refresh-terminals', {}),
+    openInExternalTerminal: (worktreePath: string, terminalId?: string) =>
+      this.post('/api/worktree/open-in-external-terminal', { worktreePath, terminalId }),
     initGit: (projectPath: string) => this.post('/api/worktree/init-git', { projectPath }),
     startDevServer: (projectPath: string, worktreePath: string) =>
       this.post('/api/worktree/start-dev', { projectPath, worktreePath }),