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/invite-user-prompt/InviteUserPrompt.tsx

@@ -261,9 +261,12 @@ export function InviteUserPrompt({ room, requestClose }: InviteUserProps) {
                 gap="400"
               >
                 <Box direction="Column" gap="100">
-                  <Text size="L400">User ID</Text>
+                  <Text as="label" htmlFor="invite-user-id" size="L400">
+                    User ID
+                  </Text>
                   <div>
                     <Input
+                      id="invite-user-id"
                       size="500"
                       ref={inputRef}
                       onChange={handleSearchChange}
@@ -334,8 +337,11 @@ export function InviteUserPrompt({ room, requestClose }: InviteUserProps) {
                   </div>
                 </Box>
                 <Box direction="Column" gap="100">
-                  <Text size="L400">Reason (Optional)</Text>
+                  <Text as="label" htmlFor="invite-reason" size="L400">
+                    Reason (Optional)
+                  </Text>
                   <TextArea
+                    id="invite-reason"
                     size="500"
                     name="reasonInput"
                     variant="Background"