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/config/config.go

@@ -22,6 +22,7 @@ type Config struct {
 	FPS         int     `json:"fps"`
 	Source      string  `json:"source"`
 	Audio       bool    `json:"audio"`
+	AudioSource uint64  `json:"audio_source"`
 	StreamIndex int     `json:"stream_index"`
 	Scale       float64 `json:"scale"`
 	// Announce is a *bool so an absent JSON key (older config files) keeps
@@ -40,6 +41,7 @@ func Default() Config {
 		FPS:         c.FPS,
 		Source:      c.Source,
 		Audio:       c.Audio,
+		AudioSource: c.AudioSource,
 		StreamIndex: c.StreamIndex,
 		Scale:       c.Scale,
 		Announce:    &announce,
@@ -59,6 +61,7 @@ func (c Config) ToFlinger() flinger.Config {
 		FPS:         c.FPS,
 		Source:      c.Source,
 		Audio:       c.Audio,
+		AudioSource: c.AudioSource,
 		StreamIndex: c.StreamIndex,
 		Scale:       c.Scale,
 		Announce:    announce,