feat(desktop-client): split autostart and keep-alive per platform

Refactors openAtLogin and keepRunningInBackground to make the macOS/Windows
divergence explicit instead of leaning on Electron's cross-platform
abstraction:

- openAtLogin on Windows now passes path: process.execPath and args: []
  so the registry HKCU Run key always points at the right binary.
- keepRunningInBackground on Windows shows a one-shot tray balloon ("省心
  推已最小化到托盘…") so first-time users notice the app is still alive;
  persisted via a private hasShownBackgroundBalloon flag so it never fires
  again. macOS preserves the dock icon and stays silent.
- Sets AppUserModelID early on Windows so tray.displayBalloon's Win10+
  Toast routing actually fires.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/desktop-client/src/main/tray.ts

@@ -47,6 +47,28 @@ function formatIssueCount(count: number): string {
   return count > 99 ? '99+' : String(count)
 }
 
+// Windows-only: tray.displayBalloon shows a system toast next to the tray
+// icon. macOS and Linux ignore this call (Electron returns silently), so the
+// guard here is just to avoid the no-op log noise on non-Windows.
+export function showTrayBalloon(title: string, content: string): void {
+  if (process.platform !== 'win32') {
+    return
+  }
+  if (!tray || tray.isDestroyed()) {
+    return
+  }
+  try {
+    tray.displayBalloon({
+      title,
+      content,
+      icon: createTrayIcon('normal'),
+      iconType: 'custom',
+    })
+  } catch (error) {
+    console.warn('[desktop-tray] displayBalloon failed', error)
+  }
+}
+
 export function updateTrayIssueIndicator(issueCount: number): void {
   if (!tray || tray.isDestroyed()) {
     return