fix: pack all frames into single blob for IndexedDB storage

Previously each frame was stored as a separate Blob in the Recording
object, causing IndexedDB to hang when writing 100+ blobs in one
transaction. Now all frames are packed into a single binary blob:
  [4B frameCount][per frame: 4B ts, 4B len, JPEG data]

DB version bumped to 2 (old store deleted on upgrade).
PlaybackPanel unpacks frames on demand with a per-recording cache.

dist/index.html

@@ -5,8 +5,8 @@
     <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-D4W4a1Z1.js"></script>
-    <link rel="stylesheet" crossorigin href="/assets/index-DuJPD2XK.css">
+    <script type="module" crossorigin src="/assets/index-CW6NDYF0.js"></script>
+    <link rel="stylesheet" crossorigin href="/assets/index-BTgh_FTp.css">
   </head>
   <body>
     <div id="app"></div>