Add system tray icon (Show/Hide/Refresh/Quit; left-click restores window)

Solves the 'lost window' problem — if the widget gets dragged off-screen
or hidden behind other apps, users can recover it via the tray.

Enables tauri 'tray-icon' feature; tray runs entirely Rust-side so no
new JS capabilities needed.

README.md

@@ -52,6 +52,9 @@ displays, refreshed every 5 minutes.
 - **⚙** — Settings (custom claude command, refresh interval, autostart, distro override).
 - **×** — quit.
 - The window is **resizable** — drag any edge.
+- **System tray** — there's a tray icon next to the Windows clock while the
+  widget is running. Left-click to bring the window back if you lose it
+  off-screen; right-click for Show / Hide / Refresh / Quit.
 
 ## Troubleshooting