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/about/+page.svelte

@@ -7,7 +7,7 @@
 
   const { data } = $props();
 
-  const stats = [
+  const stats = $derived([
     {
       icon: "icon-[ri--user-heart-line]",
       value: data.stats.viewers_count,
@@ -28,7 +28,7 @@
       value: $_("about.stats.yearsFormatted", { values: { years: 5 } }),
       label: $_("about.stats.experience"),
     },
-  ];
+  ]);
 
   const team = [
     {