fix: use singleton IndexedDB connection to prevent blocking

Replaced open/close-per-operation pattern with a single persistent
connection held for the app lifetime. This prevents the DB upgrade
(v1→v2) from being blocked by lingering connections, and eliminates
transaction contention.

Also added onblocked handler that surfaces a clear error instead of
hanging indefinitely.

dist/index.html

@@ -5,7 +5,7 @@
     <link rel="icon" href="/favicon.ico">
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <title>MJPEG Stream Viewer</title>
-    <script type="module" crossorigin src="/assets/index-CW6NDYF0.js"></script>
+    <script type="module" crossorigin src="/assets/index-B6qcBY8r.js"></script>
     <link rel="stylesheet" crossorigin href="/assets/index-BTgh_FTp.css">
   </head>
   <body>