Fix critical deadlock when switching audio sources

Problem:
When switching audio sources (USB to HDMI or vice versa), the application
would hang indefinitely. This was caused by a deadlock between Go and C
layers:

1. Main thread calls SetAudioOutputSource() → stopOutputAudio()
2. stopOutputAudio() calls outputRelay.Stop() which waits for goroutine
3. Goroutine is blocked in ReadMessage() holding Go mutex
4. ReadMessage() calls blocking C function jetkvm_audio_read_encode()
5. C function is blocked reading from ALSA device
6. Disconnect() can't acquire Go mutex to clean up
7. Deadlock: Main thread waiting for goroutine, goroutine waiting for ALSA

Solution:
Release the Go mutex BEFORE calling blocking C functions in ReadMessage()
and WriteMessage(). The C layer has its own pthread mutex protection and
handles stop requests via atomic flags. This allows:
- Disconnect() to acquire the mutex immediately
- C layer to detect stop request and return quickly
- Goroutines to exit cleanly
- Audio source switching to work flawlessly

Fixes:
- internal/audio/cgo_source.go:ReadMessage() - Release mutex before C call
- internal/audio/cgo_source.go:WriteMessage() - Release mutex before C call

This fix eliminates the hang when switching between USB and HDMI audio
sources.

internal/audio/cgo_source.go

@@ -167,17 +167,21 @@ func (c *CgoSource) IsConnected() bool {
 }
 
 func (c *CgoSource) ReadMessage() (uint8, []byte, error) {
+	// Check connection status with mutex
 	c.mu.Lock()
-	defer c.mu.Unlock()
-
 	if !c.connected {
+		c.mu.Unlock()
 		return 0, nil, fmt.Errorf("not connected")
 	}
 
 	if c.direction != "output" {
+		c.mu.Unlock()
 		return 0, nil, fmt.Errorf("ReadMessage only supported for output direction")
 	}
+	c.mu.Unlock()
 
+	// Call C function without holding mutex to avoid deadlock
+	// The C layer has its own locking and handles stop requests
 	opusSize := C.jetkvm_audio_read_encode(unsafe.Pointer(&c.opusBuf[0]))
 	if opusSize < 0 {
 		return 0, nil, fmt.Errorf("jetkvm_audio_read_encode failed: %d", opusSize)
@@ -195,16 +199,18 @@ func (c *CgoSource) ReadMessage() (uint8, []byte, error) {
 }
 
 func (c *CgoSource) WriteMessage(msgType uint8, payload []byte) error {
+	// Check connection status and validate parameters with mutex
 	c.mu.Lock()
-	defer c.mu.Unlock()
-
 	if !c.connected {
+		c.mu.Unlock()
 		return fmt.Errorf("not connected")
 	}
 
 	if c.direction != "input" {
+		c.mu.Unlock()
 		return fmt.Errorf("WriteMessage only supported for input direction")
 	}
+	c.mu.Unlock()
 
 	if msgType != ipcMsgTypeOpus {
 		return nil
@@ -218,6 +224,8 @@ func (c *CgoSource) WriteMessage(msgType uint8, payload []byte) error {
 		return fmt.Errorf("opus packet too large: %d bytes (max 1500)", len(payload))
 	}
 
+	// Call C function without holding mutex to avoid deadlock
+	// The C layer has its own locking and handles stop requests
 	rc := C.jetkvm_audio_decode_write(unsafe.Pointer(&payload[0]), C.int(len(payload)))
 	if rc < 0 {
 		return fmt.Errorf("jetkvm_audio_decode_write failed: %d", rc)