Fix Rust build errors: use setup hook for window icon, enable image-png feature

The previous approach used Builder::default_window_icon() which doesn't
exist in Tauri 2.10. Instead, set the icon via window.set_icon() in the
setup hook, and enable the "image-png" feature flag so Image::from_bytes
can decode the PNG icon at runtime.

Also change bundle identifier from "com.triple-c.app" to
"com.triple-c.desktop" to avoid conflicting with the .app bundle
extension on macOS.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/src-tauri/Cargo.toml

@@ -12,7 +12,7 @@ name = "triple-c"
 path = "src/main.rs"
 
 [dependencies]
-tauri = { version = "2", features = [] }
+tauri = { version = "2", features = ["image-png"] }
 tauri-plugin-store = "2"
 tauri-plugin-dialog = "2"
 tauri-plugin-opener = "2"