fix: resolve vite-plugin-svelte warnings

- image-viewer: replace backdrop div with button for a11y
- file-drop-zone: wrap prop check in \$effect to avoid state_referenced_locally
- about: use \$derived for stats array
- magazine: use \$derived for featuredArticle
- play: add role/keyboard support to seek bar slider; fix \$state on SvelteMap in device-mapping-dialog
- admin/videos/[id]: add <track kind="captions"> to video element

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/frontend/src/routes/magazine/+page.svelte

@@ -22,8 +22,9 @@
   $effect(() => { searchValue = data.search ?? ""; });
   let searchTimeout: ReturnType<typeof setTimeout>;
 
-  const featuredArticle =
-    data.page === 1 && !data.search && !data.category ? data.items.find((a) => a.featured) : null;
+  const featuredArticle = $derived(
+    data.page === 1 && !data.search && !data.category ? data.items.find((a) => a.featured) : null,
+  );
 
   function debounceSearch(value: string) {
     clearTimeout(searchTimeout);