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/common-settings/permissions/PowersEditor.tsx

@@ -147,6 +147,7 @@ function EditPower({ maxPower, power, tag, onSave, onClose }: EditPowerProps) {
             <Text size="L400">Name</Text>
             <Input
               name="nameInput"
+              aria-label="Power level name"
               defaultValue={tag?.name}
               placeholder="Bot"
               size="300"
@@ -160,6 +161,7 @@ function EditPower({ maxPower, power, tag, onSave, onClose }: EditPowerProps) {
             <Input
               defaultValue={power}
               name="powerInput"
+              aria-label="Power level value"
               size="300"
               variant={typeof power === 'number' ? 'SurfaceVariant' : 'Secondary'}
               radii="300"