Real-time collaboration (Yjs + WebSocket)

- Go WebSocket hub: rooms per document, broadcast updates, persist state
- Yjs integration: connect/disconnect, sync document state
- Collab toggle button in toolbar (Solo/Live)
- When Live: edits broadcast to all connected users in real-time
- Yjs state persisted to SQLite (survives server restart)
- gorilla/websocket dependency added

cmd/server/main.go

@@ -10,6 +10,7 @@ import (
 
 	"markdownhub/internal/api"
 	"markdownhub/internal/auth"
+	"markdownhub/internal/collab"
 	"markdownhub/internal/db"
 	"markdownhub/internal/files"
 )
@@ -33,8 +34,14 @@ func main() {
 
 	router := api.NewRouter(database, dataDir, secret)
 
+	// Collab WebSocket hub
+	hub := collab.NewHub(database)
+	mux := http.NewServeMux()
+	mux.Handle("/ws/collab/", http.HandlerFunc(hub.HandleWebSocket))
+	mux.Handle("/", router)
+
 	fmt.Printf("MarkdownHub listening on :%s\n", port)
-	log.Fatal(http.ListenAndServe(":"+port, router))
+	log.Fatal(http.ListenAndServe(":"+port, mux))
 }
 
 func ensureAdminUser(database *db.DB, dataDir string) {