feat: Windows taskbar overlay badge via ITaskbarList3

Adds set_badge_count Tauri command (Windows only). Uses CoCreateInstance
to get ITaskbarList3 and SetOverlayIcon to display a dynamically drawn
badge on the taskbar button. The badge is a 16x16 GDI bitmap: red circle,
white bold Segoe UI text, capped at "99+". Passing count=0 clears the
overlay. Uses windows = 0.61 (already a transitive dep via webview2-com).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src-tauri/Cargo.toml

@@ -44,6 +44,12 @@ tauri-plugin-updater = "2"
 
 [target.'cfg(target_os = "windows")'.dependencies]
 webview2-com = "0.38"
+windows = { version = "0.61", features = [
+    "Win32_Graphics_Gdi",
+    "Win32_System_Com",
+    "Win32_UI_Shell",
+    "Win32_UI_WindowsAndMessaging",
+] }
 
 [lib]
 name = "app_lib"