feat(a11y): label form controls + overlays (P3-4)

Accessible names for ~15 controls that lacked them: invite/join/create-room/
account-data/image-pack/private-note/power-level inputs (visible <label htmlFor>
where a label exists, else aria-label); the two range sliders (night-light
intensity, noise-gate threshold); the soundboard file input; media <video>
elements; and the Media Gallery (region) + Search (dialog) overlays. Hidden
notification/preview <audio> marked aria-hidden.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/components/upload-card/UploadCardRenderer.tsx

@@ -56,6 +56,7 @@ function PreviewVideo({ fileItem }: PreviewVideoProps) {
 
   return (
     <video
+      aria-label="Video attachment preview"
       style={{
         objectFit: 'contain',
         width: '100%',