feat(ui): add Open in Terminal action to worktree dropdown

Add "Open in Terminal" option to the worktree actions dropdown menu.
This opens the system terminal in the worktree directory.

Changes:
- Add openInTerminal method to http-api-client
- Add Terminal icon and menu item to worktree-actions-dropdown
- Add onOpenInTerminal prop to WorktreeTab component
- Add handleOpenInTerminal handler to use-worktree-actions hook
- Wire up handler in worktree-panel for both mobile and desktop views

Extracted from PR #558.

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

@@ -1805,6 +1805,8 @@ export class HttpApiClient implements ElectronAPI {
       this.post('/api/worktree/switch-branch', { worktreePath, branchName }),
     openInEditor: (worktreePath: string, editorCommand?: string) =>
       this.post('/api/worktree/open-in-editor', { worktreePath, editorCommand }),
+    openInTerminal: (worktreePath: string) =>
+      this.post('/api/worktree/open-in-terminal', { worktreePath }),
     getDefaultEditor: () => this.get('/api/worktree/default-editor'),
     getAvailableEditors: () => this.get('/api/worktree/available-editors'),
     refreshEditors: () => this.post('/api/worktree/refresh-editors', {}),