Implement Rust shell: tray, global hotkey, overlay window

Frameless transparent always-on-top window, tray icon with toggle/quit,
persisted global hotkey (default Ctrl+Shift+K) with safe re-registration,
single-instance --toggle IPC as Wayland hotkey fallback, close-to-tray.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src-tauri/capabilities/default.json

@@ -5,6 +5,17 @@
   "windows": ["main"],
   "permissions": [
     "core:default",
-    "opener:default"
+    "opener:default",
+    "core:window:allow-show",
+    "core:window:allow-hide",
+    "core:window:allow-set-focus",
+    "core:window:allow-is-visible",
+    "core:window:allow-start-dragging",
+    "core:window:allow-close",
+    "clipboard-manager:allow-write-text",
+    "store:default",
+    "autostart:allow-enable",
+    "autostart:allow-disable",
+    "autostart:allow-is-enabled"
   ]
 }