fix: VideoButton disabled state, PTT listener leak, TS prop errors

- VideoButton now accepts disabled prop with tooltip and visual feedback;
  PrescreenControls passes disabled=true when cameraOnJoin=false
- PTT key listener in settings tracked via ref, cleaned up on unmount,
  guarded against stacking on double-click; useCallback + useRef
- CallControls screenshare cancel button: variant Surface -> Secondary
- General.tsx Box align prop: align -> alignItems (TS2322 fix)

src/app/features/call/Controls.tsx

@@ -81,15 +81,18 @@ export function SoundButton({ enabled, onToggle }: SoundButtonProps) {
 type VideoButtonProps = {
   enabled: boolean;
   onToggle: () => void;
+  disabled?: boolean;
 };
-export function VideoButton({ enabled, onToggle }: VideoButtonProps) {
+export function VideoButton({ enabled, onToggle, disabled }: VideoButtonProps) {
   return (
     <TooltipProvider
       position="Top"
       delay={500}
       tooltip={
         <Tooltip>
-          <Text size="T200">{enabled ? 'Stop Camera' : 'Start Camera'}</Text>
+          <Text size="T200">
+            {disabled ? 'Camera disabled in settings' : enabled ? 'Stop Camera' : 'Start Camera'}
+          </Text>
         </Tooltip>
       }
     >
@@ -102,6 +105,8 @@ export function VideoButton({ enabled, onToggle }: VideoButtonProps) {
           size="400"
           onClick={() => onToggle()}
           outlined
+          disabled={disabled}
+          style={disabled ? { opacity: 0.4, cursor: 'not-allowed' } : undefined}
         >
           <Icon
             size="400"