fix: save/restore mic state on PTT mode toggle (I-4)

When enabling PTT mode, save current mic state before muting.
When disabling PTT, restore saved state instead of unconditionally unmuting.
Prevents surprising unmute if user had manually muted before switching to PTT.

src/app/features/call/CallControls.tsx

@@ -64,13 +64,16 @@ export function CallControls({ callEmbed }: CallControlsProps) {
   const microphoneRef = useRef(microphone);
   useEffect(() => { microphoneRef.current = microphone; }, [microphone]);
 
-  // Handle PTT mode toggle mid-call
+  // Handle PTT mode toggle mid-call — save/restore mic state (I-4)
   const pttModeRef = useRef(pttMode);
+  const micBeforePTTRef = useRef<boolean | null>(null);
   useEffect(() => {
     if (pttMode && !pttModeRef.current) {
+      micBeforePTTRef.current = microphoneRef.current;
       callEmbed.control.setMicrophone(false);
     } else if (!pttMode && pttModeRef.current) {
-      callEmbed.control.setMicrophone(true);
+      callEmbed.control.setMicrophone(micBeforePTTRef.current ?? true);
+      micBeforePTTRef.current = null;
     }
     pttModeRef.current = pttMode;
   }, [pttMode, callEmbed]);