feat(desktop): Windows taskbar overlay icon for unread badge

Red-dot overlay drawn as raw RGBA (no extra resource bundled).
Shown on Windows when unread count > 0, cleared when 0.
macOS keeps numeric Dock badge; Linux has no cross-DE badge API.

Bumps version 0.10.0 -> 0.10.1.

apps/desktop/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@chat-app/desktop",
-  "version": "0.10.0",
+  "version": "0.10.1",
   "private": true,
   "description": "Tauri v2 desktop client (Windows / macOS / Linux)",
   "type": "module",