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/settings/general/General.tsx

@@ -531,6 +531,7 @@ function Appearance() {
               Intensity: {nightLightOpacity}%
             </Text>
             <input
+              aria-label="Night light intensity"
               type="range"
               min={5}
               max={80}
@@ -1663,6 +1664,7 @@ function Calls() {
                     <Text size="T200">{callDenoiseGateThreshold} dB</Text>
                   </Box>
                   <input
+                    aria-label="Noise gate threshold"
                     type="range"
                     min="-100"
                     max="0"