fix: use canvas capture for recording, remove cancel button

Recording now creates an offscreen <img> (browser-native MJPEG
decoding) and captures frames via canvas.toBlob() at 100ms intervals.
This replaces the broken fetch-based multipart parser.

- useRecorder: img+canvas capture instead of fetch+ReadableStream
- RecorderControls: removed Cancel button, only Record/Stop remain
- App.vue: removed cancel wiring

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-BqiYluth.js"></script>
-    <link rel="stylesheet" crossorigin href="/assets/index-CJmYwh3k.css">
+    <script type="module" crossorigin src="/assets/index-3GYuRNvV.js"></script>
+    <link rel="stylesheet" crossorigin href="/assets/index-DuJPD2XK.css">
   </head>
   <body>
     <div id="app"></div>