feat(desktop): unread-chat count badge on the taskbar icon

Mirrors the rail's unread-chat count onto the Windows taskbar icon (overlay) and the
macOS/Linux dock badge. updateRailUnread() draws a small red count badge on a canvas
and hands it to the shell via bizConnectNative.setUnread(count, dataUrl); main sets it
with win.setOverlayIcon + app.setBadgeCount. Clears at 0, shows 99+ past 99. No-op in a
browser/mobile. (home.html build batch17)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

desktop/preload.js

@@ -10,4 +10,7 @@ contextBridge.exposeInMainWorld('bizConnectNative', Object.freeze({
   // Bring the app window to the foreground (e.g. when a notification is clicked) — the web
   // Notification's window.focus() can't raise an Electron window; the main process must.
   focusApp: () => ipcRenderer.send('focus-window'),
+  // Show the unread-chat count as a badge on the taskbar icon. count=number of chats with
+  // unread; dataUrl=a small PNG badge the renderer drew (null to clear).
+  setUnread: (count, dataUrl) => ipcRenderer.send('set-unread', { count, dataUrl }),
 }));