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/join-address-prompt/JoinAddressPrompt.tsx

@@ -108,8 +108,11 @@ export function JoinAddressPrompt({ onOpen, onCancel }: JoinAddressProps) {
                 </Text>
               </Box>
               <Box direction="Column" gap="100">
-                <Text size="L400">Address</Text>
+                <Text as="label" htmlFor="join-address" size="L400">
+                  Address
+                </Text>
                 <Input
+                  id="join-address"
                   size="500"
                   autoFocus
                   name="addressInput"