feat(mana-sync): unified WebSocket — one connection per user instead of 27

Add unified /ws endpoint that serves all app notifications over a single connection.
The server now includes appId in the sync-available message payload so the client
knows which app to pull. Legacy /ws/{appId} endpoint remains for backward compatibility.

Backend (Go):
- hub.go: Message struct gains AppId field, NotifyUser sends to all user clients
  (unified clients receive everything, legacy clients filtered by appId)
- main.go: new GET /ws route (empty appId = unified mode)

Frontend (sync.ts):
- Single connectUnifiedWs() replaces 27 per-app connectWs() calls
- Parses msg.appId from server to pull only the affected app
- Reconnect/offline logic simplified to one WS

This reduces WebSocket connections from 27 per user to 1, cutting server
connection overhead by ~96%.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

services/mana-sync/cmd/server/main.go

@@ -59,7 +59,12 @@ func main() {
 	mux.HandleFunc("POST /sync/{appId}", handler.HandleSync)
 	mux.HandleFunc("GET /sync/{appId}/pull", handler.HandlePull)
 
-	// WebSocket endpoint
+	// WebSocket endpoints
+	// Unified: one connection per user, receives all app notifications with appId in payload
+	mux.HandleFunc("/ws", func(w http.ResponseWriter, r *http.Request) {
+		hub.HandleWebSocket(w, r, "") // empty appID = unified mode
+	})
+	// Legacy: one connection per app (backward-compatible)
 	mux.HandleFunc("/ws/{appId}", func(w http.ResponseWriter, r *http.Request) {
 		appID := r.PathValue("appId")
 		hub.HandleWebSocket(w, r, appID)