fix(a11y): name FilesPage drop zone region and denest Card

src/lib/pages/FilesPage.svelte

@@ -152,16 +152,19 @@
   <!-- Title -->
   <h2 class="font-display text-[26px] italic text-text px-7 pt-6 pb-4">File Transcription</h2>
 
-  <!-- Drop zone -->
+  <!-- Drop zone. The dashed border IS the affordance, so we drop the
+       outer Card (which doubled the surface treatment per brand spec)
+       and let the dashed div be the single surface. The region landmark
+       carries an explicit accessible name per WCAG ARIA13. -->
   <div class="px-7 pb-3">
-    <Card tone="subtle">
     <div
-        class="flex flex-col items-center justify-center py-10 px-6 rounded-xl border-2 border-dashed m-3
+      class="flex flex-col items-center justify-center py-10 px-6 rounded-xl border-2 border-dashed
         {isDragOver
           ? 'border-accent bg-accent-subtle scale-[1.01]'
           : 'border-border hover:border-text-tertiary'}"
       style="transition-duration: var(--duration-ui)"
       role="region"
+      aria-label="Audio file drop zone"
     >
       {#if !fileTranscript && !transcribing}
         <EmptyState
@@ -180,7 +183,6 @@
         MP3, WAV, M4A, MP4, FLAC, OGG
       </p>
     </div>
-    </Card>
   </div>
 
   <!-- Browse buttons + progress -->