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/features/room/PollCreator.tsx

@@ -142,7 +142,6 @@ export function PollCreator({ roomId, onClose }: PollCreatorProps) {
                   placeholder="Ask a question…"
                   value={question}
                   onChange={(e: React.ChangeEvent<HTMLInputElement>) => setQuestion(e.target.value)}
-                  // eslint-disable-next-line jsx-a11y/no-autofocus
                   autoFocus
                 />
               </Box>
@@ -151,7 +150,6 @@ export function PollCreator({ roomId, onClose }: PollCreatorProps) {
               <Box direction="Column" gap="200">
                 <Text size="L400">Options</Text>
                 {options.map((opt, index) => (
-                  // eslint-disable-next-line react/no-array-index-key
                   <Box key={index} alignItems="Center" gap="200">
                     <Input
                       style={{ flex: 1 }}