fix(ui): note join/leave sound preview affordance (N82)

Selecting a join/leave sound auto-plays a preview, but nothing communicated
that. Add it to the SettingTile description.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/features/settings/general/General.tsx

@@ -1587,7 +1587,7 @@ function Calls() {
       <SequenceCard className={SequenceCardStyle} variant="SurfaceVariant" direction="Column">
         <SettingTile
           title="Join & Leave Sounds"
-          description="Play a sound when someone joins or leaves a call you are in."
+          description="Play a sound when someone joins or leaves a call you are in. Selecting an option plays a preview."
           after={
             <SettingsSelect
               value={callJoinLeaveSound}