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/AboutDialog.svelte

@@ -12,8 +12,25 @@
   });
 </script>
 
-<div class="overlay" role="presentation" onclick={onClose}>
-  <div class="dialog" role="dialog" onclick={(e) => e.stopPropagation()}>
+<div
+  class="overlay"
+  role="presentation"
+  tabindex="-1"
+  onclick={onClose}
+  onkeydown={(e) => {
+    if (e.key === 'Escape' || e.key === 'Enter' || e.key === ' ') {
+      if (e.key !== 'Escape') e.preventDefault();
+      onClose();
+    }
+  }}
+>
+  <div
+    class="dialog"
+    role="dialog"
+    tabindex="-1"
+    onclick={(e) => e.stopPropagation()}
+    onkeydown={(e) => e.stopPropagation()}
+  >
     <img class="app-icon" src="/favicon.svg" alt="GUI Video Clipper icon" />
     <h2>GUI Video Clipper</h2>
     {#if version}