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/lib/components/ui/file-drop-zone/file-drop-zone.svelte

@@ -23,11 +23,13 @@
     ...rest
   }: FileDropZoneProps = $props();
 
-  if (maxFiles !== undefined && fileCount === undefined) {
-    console.warn(
-      "Make sure to provide FileDropZone with `fileCount` when using the `maxFiles` prompt",
-    );
-  }
+  $effect(() => {
+    if (maxFiles !== undefined && fileCount === undefined) {
+      console.warn(
+        "Make sure to provide FileDropZone with `fileCount` when using the `maxFiles` prompt",
+      );
+    }
+  });
 
   let uploading = $state(false);