feat: Add Help menu with Check for Updates action

Menu implementation:
- Added menu bar to MainWindow
- Help menu with 'Check for Updates...' action
- Signal-based architecture for async update check
- Action triggers check_for_updates signal

Test coverage:
- 3 new tests for menu bar functionality
- Test menu bar creation
- Test signal exists and is callable
- Test callback method exists
- All 146 tests passing, 86% coverage

Design:
- Decoupled menu from update logic
- Emits signal that main app listens to
- Allows async update check without blocking UI

src/webdrop_bridge/ui/main_window.py

@@ -3,7 +3,7 @@
 from pathlib import Path
 from typing import Optional
 
-from PySide6.QtCore import QSize, Qt, QUrl
+from PySide6.QtCore import QSize, Qt, QUrl, Signal
 from PySide6.QtWidgets import QMainWindow, QToolBar, QVBoxLayout, QWidget
 
 from webdrop_bridge.config import Config
@@ -173,6 +173,9 @@ class MainWindow(QMainWindow):
     integration with the native filesystem.
     """
 
+    # Signals
+    check_for_updates = Signal()
+
     def __init__(
         self,
         config: Config,
@@ -202,6 +205,9 @@ class MainWindow(QMainWindow):
         # Create navigation toolbar (Kiosk-mode navigation)
         self._create_navigation_toolbar()
 
+        # Create menu bar
+        self._create_menu_bar()
+
         # Create drag interceptor
         self.drag_interceptor = DragInterceptor()
 
@@ -341,6 +347,25 @@ class MainWindow(QMainWindow):
         )
         toolbar.addAction(refresh_action)
 
+    def _create_menu_bar(self) -> None:
+        """Create menu bar with Help menu and update check action."""
+        menu_bar = self.menuBar()
+        
+        # Help menu
+        help_menu = menu_bar.addMenu("Help")
+        
+        # Check for Updates action
+        check_updates_action = help_menu.addAction("Check for Updates...")
+        check_updates_action.triggered.connect(self._on_check_for_updates)
+
+    def _on_check_for_updates(self) -> None:
+        """Handle check for updates menu action.
+        
+        Emits the check_for_updates signal to allow the main application
+        to perform the update check asynchronously.
+        """
+        self.check_for_updates.emit()
+
     def _navigate_home(self) -> None:
         """Navigate to the home (start) URL."""
         home_url = self.config.webapp_url