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/tauri.conf.json

@@ -2,7 +2,7 @@
   "$schema": "https://raw.githubusercontent.com/tauri-apps/tauri/dev/crates/tauri-cli/schema.json",
   "productName": "Triple-C",
   "version": "0.1.0",
-  "identifier": "com.triple-c.app",
+  "identifier": "com.triple-c.desktop",
   "build": {
     "beforeDevCommand": "npm run dev",
     "devUrl": "http://localhost:1420",