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/VoiceMessageRecorder.tsx

@@ -282,7 +282,12 @@ export function VoiceMessageRecorder({ onSend, onError }: VoiceRecorderProps) {
     >
       {previewUrl && (
         <>
-          <audio ref={previewAudioRef} src={previewUrl} onEnded={() => setPreviewPlaying(false)} />
+          <audio
+            ref={previewAudioRef}
+            src={previewUrl}
+            onEnded={() => setPreviewPlaying(false)}
+            aria-hidden="true"
+          />
           <IconButton
             onClick={() => {
               const audio = previewAudioRef.current;