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/components.css

@@ -3,6 +3,9 @@
 #app {
   width: 100%;
   height: 100%;
+  overflow: hidden;
+  border-radius: var(--radius-window);
+  background: var(--bg-app);
 }
 
 .layout {
@@ -11,6 +14,7 @@
   grid-template-columns: 320px minmax(0, 1fr);
   width: 100%;
   height: 100%;
+  background: var(--bg-app);
 }
 
 /* ── Sidebar ──────────────────────────────────────────────────────────────── */