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/App.svelte

@@ -188,9 +188,23 @@
       <div
         class="resize-handle"
         class:active={isResizing}
-        role="separator"
+        role="slider"
         aria-orientation="horizontal"
+        aria-label="Resize timeline and clip list"
+        aria-valuenow={timelineHeight}
+        aria-valuemin={40}
+        aria-valuemax={500}
+        tabindex="0"
         onmousedown={handleResizeStart}
+        onkeydown={(e) => {
+          if (e.key === 'ArrowDown') {
+            e.preventDefault();
+            timelineHeight = Math.min(500, timelineHeight + 10);
+          } else if (e.key === 'ArrowUp') {
+            e.preventDefault();
+            timelineHeight = Math.max(40, timelineHeight - 10);
+          }
+        }}
       ></div>
       <div class="cliplist-pane">
         <ClipList onExport={() => (showExportDialog = true)} />