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/tauri.conf.json

@@ -1,6 +1,6 @@
 {
   "$schema": "https://schema.tauri.app/config/2",
-  "productName": "kcd2-overlay",
+  "productName": "KCD2 Overlay",
   "version": "0.1.0",
   "identifier": "sk.ecoposta.kcd2overlay",
   "build": {
@@ -12,9 +12,18 @@
   "app": {
     "windows": [
       {
-        "title": "kcd2-overlay",
-        "width": 800,
-        "height": 600
+        "label": "main",
+        "title": "KCD2 Overlay",
+        "width": 920,
+        "height": 640,
+        "minWidth": 560,
+        "minHeight": 400,
+        "decorations": false,
+        "transparent": true,
+        "alwaysOnTop": true,
+        "skipTaskbar": true,
+        "visible": false,
+        "center": true
       }
     ],
     "security": {