feat(start-menu): add context menu with pin/unpin for apps

[PrayagCodes]

Summary

Adds right-click context menu support for apps in the start menu, allowing users to launch apps and pin/unpin them to the taskbar directly from the start menu without needing to launch the app first.

Problem

Previously, users could only pin apps to the taskbar by:

1. Launching the app first
2. Then right-clicking the app's taskbar item to pin it

This created unnecessary friction for users who wanted to organize their taskbar without launching apps.

Solution

This PR adds a context menu to start menu apps that provides:

• Open option to launch the app
• Keep in Taskbar / Remove from Taskbar options based on current pin state
• Full pin/unpin functionality that persists across sessions
• Mobile support via long-press gesture

Changes

Frontend (src/gui/src/UI/UIDesktop.js)

• ✅ Added context menu event handler for .start-app elements (right-click/taphold)
• ✅ Implemented isAppPinned() helper function to check pinned state
• ✅ Implemented pinAppToTaskbar() function to add apps to taskbar
• ✅ Implemented unpinAppFromTaskbar() function to remove apps from taskbar
• ✅ Fixed state synchronization in update_taskbar() to keep in-memory state in sync with DOM
• ✅ Added UITaskbarItem import

Backend (src/backend/src/routers/update-taskbar-items.js)

• ✅ Fixed database persistence by properly stringifying taskbar items array

Technical Details

• Uses existing UIContextMenu component for consistency
• Follows same pin/unpin patterns as taskbar item context menus
• Closes existing context menus before opening new ones to prevent multiple menus
• Synchronizes window.user.taskbar_items with DOM state
• Handles edge cases (apps with open windows, already pinned apps, etc.)

Testing

Manual Testing Checklist

• Right-click unpinned app shows "Open" and "Keep in Taskbar"
• Right-click pinned app shows "Open" and "Remove from Taskbar"
• Clicking "Keep in Taskbar" adds app to taskbar immediately
• Clicking "Remove from Taskbar" removes app from taskbar (if no windows open)
• Unpinning app with open windows keeps it in taskbar until windows close
• Changes persist after page refresh
• Only one context menu appears at a time
• Context menu closes when clicking outside
• Context menu closes when selecting "Open"
• Long-press works on mobile devices

Test Scenarios

1. Pin from start menu

   • Right-click unpinned app → Click "Keep in Taskbar"
   • ✅ App appears in taskbar immediately
   • ✅ Refresh page → App remains pinned

2. Unpin from start menu (no windows)

   • Right-click pinned app → Click "Remove from Taskbar"
   • ✅ App disappears from taskbar immediately
   • ✅ Refresh page → App remains unpinned

3. Unpin with windows open

   • Launch app → Right-click in start menu → Click "Remove from Taskbar"
   • ✅ App stays in taskbar (window is open)
   • ✅ Close window → App disappears from taskbar

4. Multiple menus prevention

   • Right-click app A → Right-click app B
   • ✅ First menu closes, second menu opens