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/user-profile/UserRoomProfile.tsx

@@ -253,6 +253,7 @@ function UserPrivateNotes({ userId }: { userId: string }) {
         )}
       </Box>
       <textarea
+        aria-label="Private note about this user"
         value={draft}
         onChange={handleChange}
         maxLength={USER_NOTE_MAX_LENGTH}