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/MediaGallery.tsx

@@ -186,8 +186,8 @@ function LightboxMedia({
       )}
       {media.status === 'ok' &&
         (item.msgtype === MsgType.Video ? (
-          // eslint-disable-next-line jsx-a11y/media-has-caption
           <video
+            aria-label="Video attachment"
             src={media.url}
             controls
             autoPlay
@@ -261,7 +261,7 @@ function Lightbox({
           escapeDeactivates: false,
         }}
       >
-        {/* eslint-disable-next-line jsx-a11y/no-static-element-interactions */}
+        {}
         <div
           role="dialog"
           aria-modal
@@ -640,13 +640,15 @@ export function MediaGallery({ room, onClose }: MediaGalleryProps) {
         className={classNames(css.MediaGalleryDrawer, ContainerColor({ variant: 'Background' }))}
         shrink="No"
         direction="Column"
+        role="region"
+        aria-labelledby="media-gallery-title"
       >
         {/* Header */}
         <Header variant="Background" size="600" className={css.MediaGalleryHeader}>
           <Box grow="Yes" alignItems="Center" gap="200">
             <Icon size="200" src={Icons.Photo} />
             <Box grow="Yes">
-              <Text size="H4" truncate>
+              <Text id="media-gallery-title" size="H4" truncate>
                 Media Gallery
               </Text>
             </Box>