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/Cargo.lock

@@ -96,6 +96,8 @@ version = "0.1.0"
 dependencies = [
  "dirs 5.0.1",
  "log",
+ "objc2 0.5.2",
+ "objc2-app-kit 0.2.2",
  "serde",
  "serde_json",
  "tauri",