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/tauri.conf.json

@@ -8,6 +8,7 @@
   },
   "app": {
     "withGlobalTauri": true,
+    "macOSPrivateApi": true,
     "windows": [
       {
         "title": "Gitpub Desktop",
@@ -15,7 +16,8 @@
         "height": 860,
         "minWidth": 1024,
         "minHeight": 680,
-        "decorations": false
+        "decorations": false,
+        "transparent": true
       }
     ],
     "security": {