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/play/+page.svelte

@@ -480,12 +480,22 @@
                   .padStart(2, "0")}
               </span>
               <div
+                role="slider"
+                tabindex="0"
+                aria-label="Seek"
+                aria-valuemin={0}
+                aria-valuemax={data.recording.duration}
+                aria-valuenow={playbackProgress}
                 class="flex-1 h-2 bg-muted rounded-full overflow-hidden cursor-pointer relative"
                 onclick={(e) => {
                   const rect = e.currentTarget.getBoundingClientRect();
                   const percentage = ((e.clientX - rect.left) / rect.width) * 100;
                   seek(percentage);
                 }}
+                onkeydown={(e) => {
+                  if (e.key === "ArrowRight") seek(((playbackProgress + 1) / data.recording.duration) * 100);
+                  else if (e.key === "ArrowLeft") seek(((playbackProgress - 1) / data.recording.duration) * 100);
+                }}
               >
                 <div
                   class="absolute inset-0 bg-gradient-to-r from-primary to-accent transition-all duration-150"

packages/frontend/src/routes/play/components/device-mapping-dialog.svelte

@@ -16,7 +16,7 @@
   let { open, recordedDevices, connectedDevices, onConfirm, onCancel }: Props = $props();
 
   // Device mappings: recorded device name -> connected device
-  let mappings = new SvelteMap<string, BluetoothDevice>();
+  let mappings = $state(new SvelteMap<string, BluetoothDevice>());
 
   // Check if a connected device is compatible with a recorded device
   function isCompatible(recordedDevice: DeviceInfo, connectedDevice: BluetoothDevice): boolean {