feat(macos): implement transparent titlebar with custom background color

- Add transparent titlebar configuration in tauri.conf.json
- Implement macOS titlebar background color matching main UI banner (#3498db)
- Replace deprecated cocoa crate with modern objc2-app-kit
- Preserve native window functionality (drag, traffic lights)
- Remove all deprecation warnings from build process

The titlebar now seamlessly matches the application's blue theme while
maintaining all native macOS window management features.

src-tauri/tauri.conf.json

@@ -18,7 +18,8 @@
         "minWidth": 800,
         "minHeight": 600,
         "resizable": true,
-        "fullscreen": false
+        "fullscreen": false,
+        "titleBarStyle": "Transparent"
       }
     ],
     "security": {