feat: selectable audio source

Add an audio source picker to the GUI so users can capture a specific
PipeWire device (sink monitor or microphone) instead of only the system
default output.

- vendor go2tv.app/screencast and patch the audio stream to accept a
  target PipeWire node serial (PW_KEY_TARGET_OBJECT); the upstream lib
  only ever auto-connected to the default
- capture: ListAudioSources enumerates PipeWire sinks/sources via pw-dump;
  OpenPipeWire takes the selected node serial
- flinger/config/gui: AudioSource config field, persisted and exposed as
  an Audio source dropdown (default output + enumerated devices)

Also fixes two pre-existing bugs surfaced by stop/start testing:
- engine Stop now waits for the video/audio/stats goroutines before
  destroying the encoder (was a use-after-free SIGSEGV)
- Start/Stop now pause/resume the engine instead of tearing down and
  re-opening the portal session, which the portal cannot reliably do
  in-process (2nd CreateSession returned Ended/cancelled). Capture
  session stays open across stop/start.

internal/flinger/flinger_test.go

@@ -22,7 +22,7 @@ func TestEnginePatternStartStop(t *testing.T) {
 	}
 
 	eng.Start()
-	defer eng.Stop()
+	defer eng.Close()
 
 	// Connect a raw receiver and read until the engine reports frames sent.
 	conn, err := net.Dial("tcp", "127.0.0.1:"+itoa(eng.sender.Port()))
@@ -143,7 +143,7 @@ func TestBitrateMeasurement(t *testing.T) {
 		t.Fatalf("New: %v", err)
 	}
 	eng.Start()
-	defer eng.Stop()
+	defer eng.Close()
 
 	// Connect a receiver so packets actually flow, and wait for the bitrate
 	// window to produce a measurement.