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/search/Search.tsx

@@ -247,7 +247,6 @@ export function Search({ requestClose }: SearchProps) {
         <FocusTrap
           focusTrapOptions={{
             initialFocus: () => inputRef.current,
-            returnFocusOnDeactivate: false,
             allowOutsideClick: true,
             clickOutsideDeactivates: true,
             onDeactivate: requestClose,
@@ -257,7 +256,13 @@ export function Search({ requestClose }: SearchProps) {
             },
           }}
         >
-          <Modal size="400" style={{ maxHeight: toRem(400), borderRadius: config.radii.R500 }}>
+          <Modal
+            size="400"
+            role="dialog"
+            aria-modal="true"
+            aria-label="Search"
+            style={{ maxHeight: toRem(400), borderRadius: config.radii.R500 }}
+          >
             <Box
               shrink="No"
               style={{ padding: config.space.S400, paddingBottom: 0 }}
@@ -270,6 +275,7 @@ export function Search({ requestClose }: SearchProps) {
                 radii="400"
                 outlined
                 placeholder="Search"
+                aria-label="Search rooms"
                 before={<Icon size="200" src={Icons.Search} />}
                 onChange={handleInputChange}
                 onKeyDown={handleInputKeyDown}