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.

src/composables/useRecorder.ts

@@ -1,5 +1,5 @@
 import { ref, type Ref } from 'vue'
-import { saveRecording, type Recording } from './useStorage'
+import { saveRecording, packFrames, type Recording } from './useStorage'
 
 export type RecorderState = 'idle' | 'recording' | 'saving'
 
@@ -161,7 +161,7 @@ export function useRecorder() {
       createdAt: new Date(),
       duration: elapsedMs.value,
       frameCount: savedFrames.length,
-      frames: savedFrames,
+      data: packFrames(savedFrames),
     }
 
     try {