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/create-room/CreateRoomAliasInput.tsx

@@ -78,11 +78,14 @@ export function CreateRoomAliasInput({ disabled }: { disabled?: boolean }) {
 
   return (
     <Box shrink="No" direction="Column" gap="100">
-      <Text size="L400">Address (Optional)</Text>
+      <Text as="label" htmlFor="create-room-alias" size="L400">
+        Address (Optional)
+      </Text>
       <Text size="T200" priority="300">
         Pick an unique address to make it discoverable.
       </Text>
       <Input
+        id="create-room-alias"
         ref={aliasInputRef}
         onChange={handleAliasChange}
         before={