refactor: stop recording triggers file download instead of browser storage

Recording flow: click Stop → frames packed into .mjpeg blob → browser
download triggered immediately. No more IndexedDB or OPFS.

Playback: use 'Load Recording File' button to open a .mjpeg file.
Removed all storage dependencies — pure file-based, 100% reliable.

src/components/RecorderControls.vue

@@ -35,16 +35,10 @@ const emit = defineEmits<{
         v-if="state === 'recording'"
         class="btn btn-stop"
         @click="emit('stop')"
-        title="Stop and save recording"
+        title="Stop and download recording"
       >
         <span class="btn-icon">⏹</span>
-        Stop &amp; Save
-      </button>
-
-      <!-- Saving -->
-      <button v-if="state === 'saving'" class="btn btn-saving" disabled>
-        <span class="spinner"></span>
-        Saving…
+        Stop &amp; Download
       </button>
     </div>
 
@@ -119,25 +113,6 @@ const emit = defineEmits<{
   background: #c0392b;
 }
 
-.btn-saving {
-  background: #666;
-  color: #fff;
-}
-
-.spinner {
-  display: inline-block;
-  width: 14px;
-  height: 14px;
-  border: 2px solid rgba(255, 255, 255, 0.3);
-  border-top-color: #fff;
-  border-radius: 50%;
-  animation: spin 0.6s linear infinite;
-}
-
-@keyframes spin {
-  to { transform: rotate(360deg); }
-}
-
 .recording-info {
   display: flex;
   align-items: center;