config(tauri): simplify icon configuration to use unified PNG format

- Replace platform-specific icon formats (icns, ico) with single icon.png
- Add icon_template.ico for development reference
- Streamlines build process across platforms

src-tauri/tauri.conf.json

@@ -38,8 +38,7 @@
       "icons/32x32.png",
       "icons/128x128.png",
       "icons/128x128@2x.png",
-      "icons/icon.icns",
-      "icons/icon.ico"
+      "icons/icon.png"
     ]
   }
 }