Enable transparent macOS window and UI tweaks

Enable macOS private API and make the app window transparent. Add tauri feature and tauri.conf settings (macOSPrivateApi + transparent) and call set_background_color in setup to use an RGBA transparent background. Adjust CSS so body is transparent, add --radius-window, and move app background to #app/.layout with border-radius and overflow hidden to preserve rounded transparent window styling.

src-tauri/Cargo.toml

@@ -18,7 +18,7 @@ crate-type = ["staticlib", "cdylib", "rlib"]
 tauri-build = { version = "2", features = [] }
 
 [dependencies]
-tauri = { version = "2", features = [] }
+tauri = { version = "2", features = ["macos-private-api"] }
 tauri-plugin-opener = "2"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"