feat(macos): implement native PlatformMenuBar for macOS system menu (#612)

[ZhuchkaTriplesix]

Summary

Implements native macOS top system menu bar integration via Flutter's PlatformMenuBar.

Changes

1. QueryaPlatformMenuBar (lib/features/macos/querya_platform_menu_bar.dart):
   • Application Menu: About Querya, Preferences (Cmd+,), Hide (Cmd+H), Hide Others (Cmd+Alt+H), Show All, Quit (Cmd+Q).
   • File Menu: New Connection (Cmd+N), New Query Tab (Cmd+Shift+N), Open SQL Script (Cmd+O), Save Query (Cmd+S), Run Query / Script (Cmd+Enter), Close Tab (Cmd+W).
   • Edit Menu: Undo (Cmd+Z), Redo (Cmd+Shift+Z), Cut (Cmd+X), Copy (Cmd+C), Paste (Cmd+V), Select All (Cmd+A).
   • View Menu: Toggle Sidebar (Cmd+B), Compound Filter Bar (Cmd+F), Groupings & Pivot Drawer (Cmd+G), Return to Start Screen (Cmd+Shift+0), Full Screen.
   • Window Menu: Minimize, Zoom.
   • Help Menu: Welcome Tour & Guide (F1), Keyboard Shortcuts Cheat Sheet (Cmd+Shift+H), Querya GitHub Repository & Bug Report URLs.
2. MainScreen Integration (lib/features/main_screen/main_screen.dart):
   • Wrapped MainScreen with QueryaPlatformMenuBar binding global intents and bridge callbacks.
3. Tests:
   • Widget tests in test/features/macos/querya_platform_menu_bar_test.dart validating full menu structure, keyboard shortcut activators, and callback routing.

Closes #612