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.

frontend/css/base.css

@@ -27,6 +27,7 @@
   /* Shape */
   --radius-md: 7px;
   --radius-lg: 11px;
+  --radius-window: 14px;
   --shadow: 0 8px 24px rgba(0, 0, 0, 0.35);
 }
 
@@ -58,7 +59,7 @@ body {
   margin: 0;
   width: 100%;
   height: 100%;
-  background: var(--bg-app);
+  background: transparent;
   color: var(--text-main);
   font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", "Noto Sans", Helvetica, Arial, sans-serif;
   font-size: 14px;