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/capture/audio.go

@@ -0,0 +1,97 @@
+// Audio source enumeration for the settings UI.
+
+package capture
+
+import (
+	"encoding/json"
+	"os/exec"
+)
+
+// AudioDevice describes a capturable PipeWire audio node.
+type AudioDevice struct {
+	Serial   uint64 // PipeWire object.serial, passed to the capture backend
+	ID       uint32 // PipeWire node id (informational)
+	Name     string // node name, e.g. "alsa_output...analog-stereo"
+	Desc     string // human-readable description
+	IsOutput bool   // true = a sink (system output); false = a source (mic)
+}
+
+// pwDumpNode is the subset of `pw-dump` output we parse.
+type pwDumpNode struct {
+	ID   uint32 `json:"id"`
+	Info struct {
+		Props map[string]any `json:"props"`
+	} `json:"info"`
+}
+
+// ListAudioSources enumerates PipeWire audio sinks and sources via `pw-dump`.
+// To capture system audio we attach to a sink's monitor; to capture a
+// microphone we attach to an Audio/Source node. This lists the capturable
+// audio nodes so the GUI can present a picker instead of always using the
+// system default output.
+func ListAudioSources() ([]AudioDevice, error) {
+	out, err := exec.Command("pw-dump").Output()
+	if err != nil {
+		return nil, err
+	}
+
+	var nodes []pwDumpNode
+	if err := json.Unmarshal(out, &nodes); err != nil {
+		return nil, err
+	}
+
+	var sources []AudioDevice
+	for _, n := range nodes {
+		props := n.Info.Props
+		mediaClass, _ := props["media.class"].(string)
+		if !audioNodeClass(mediaClass) {
+			continue
+		}
+		desc, _ := props["node.description"].(string)
+		name, _ := props["node.name"].(string)
+		serial := toUint64(props["object.serial"])
+		if serial == 0 {
+			continue
+		}
+		sources = append(sources, AudioDevice{
+			Serial:   serial,
+			ID:       n.ID,
+			Name:     name,
+			Desc:     desc,
+			IsOutput: mediaClass == "Audio/Sink",
+		})
+	}
+	return sources, nil
+}
+
+// audioNodeClass reports whether a media.class is a capturable audio node.
+func audioNodeClass(mediaClass string) bool {
+	switch mediaClass {
+	case "Audio/Sink", "Audio/Source":
+		return true
+	default:
+		return false
+	}
+}
+
+// toUint64 best-effort converts a pw-dump property value to uint64.
+// PipeWire serials are small, so the conversions cannot overflow in practice.
+//
+//nolint:gosec // safe: JSON numbers from pw-dump are small node serials
+func toUint64(v any) uint64 {
+	switch t := v.(type) {
+	case float64:
+		return uint64(t)
+	case int:
+		return uint64(t)
+	case int64:
+		return uint64(t)
+	case uint64:
+		return t
+	case json.Number:
+		if n, err := t.Int64(); err == nil {
+			return uint64(n)
+		}
+	}
+	return 0
+}