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/soundboard-pack-view/SoundboardPackEditor.tsx

@@ -278,6 +278,7 @@ export function SoundboardPackEditor({ pack, canEdit, onUpdate }: SoundboardPack
     <Box direction="Column" gap="300">
       <input
         ref={fileInputRef}
+        aria-label="Upload soundboard clip"
         type="file"
         accept={SOUNDBOARD_ACCEPT}
         multiple