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.toml

@@ -26,3 +26,7 @@ tauri-plugin-log = "2"
 tauri-plugin-shell = "2"
 tauri-plugin-opener = "2"
 dirs = "5.0"
+
+[target.'cfg(target_os = "macos")'.dependencies]
+objc2 = "0.5"
+objc2-app-kit = { version = "0.2", features = ["NSColor"] }