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/admin/videos/[id]/+page.svelte

@@ -154,7 +154,9 @@
           poster={imageId ? (getAssetUrl(imageId, "preview") ?? undefined) : undefined}
           controls
           class="w-full rounded-lg bg-black max-h-72 mb-2"
-        ></video>
+        >
+          <track kind="captions" />
+        </video>
       {/if}
       <FileDropZone accept="video/*" maxFileSize={2000 * MEGABYTE} onUpload={handleVideoUpload} />
     </div>