fix: resolve all Svelte a11y warnings

- Associate labels with controls in CaptionSettingsPanel (9 warnings)
- Add tabindex and keyboard handlers to dialog overlays (8 warnings)
- Add <track> to video element for caption accessibility
- Add keyboard support to resize handle separator

Co-authored-by: Cursor <cursoragent@cursor.com>

src/lib/components/ClipList.svelte

@@ -43,7 +43,17 @@
   }
 </script>
 
-<div class="clip-list" role="listbox" onclick={handleContainerClick}>
+<div
+  class="clip-list"
+  role="listbox"
+  tabindex="0"
+  onclick={handleContainerClick}
+  onkeydown={(e) => {
+    if (e.key === 'Escape') {
+      selectClip(null);
+    }
+  }}
+>
   {#if clips.length === 0}
     <div class="empty">No clips yet — press I to mark in-point, O to mark out-point</div>
   {:else}