feat: live settings changes mid-stream

Add flinger.SetConfig to update FPS/quality/name/audio/announce without
restarting the stream. Fields that require a restart (source, port,
stream-index) are preserved. The video loop reads the live config each
frame so changes apply immediately, and the GUI Quality/Frame-rate
dropdowns push changes to a running engine.

Verified live: fps 12->33 and JPEG size 210KB->1.3MB on quality change.

internal/flinger/flinger.go

@@ -99,7 +99,8 @@ type Status struct {
 
 // Engine owns the capture, encode and send pipeline.
 type Engine struct {
-	cfg Config
+	cfgMu sync.RWMutex
+	cfg   Config
 
 	sender    *output.Sender
 	announcer *discovery.Announcer
@@ -122,6 +123,35 @@ func (e *Engine) setErr(err error) {
 	e.errMu.Unlock()
 }
 
+// SetConfig updates engine settings live (FPS, quality, name, etc.) without
+// restarting the stream. It validates the new config first; on error the
+// engine keeps its current settings.
+//
+// Not every field is live-applicable mid-stream: source, port and
+// stream-index still require a restart (they are ignored if changed).
+func (e *Engine) SetConfig(cfg Config) error {
+	if err := cfg.Validate(); err != nil {
+		return err
+	}
+	e.cfgMu.Lock()
+	defer e.cfgMu.Unlock()
+
+	// Fields that cannot change live keep their current values.
+	cfg.Source = e.cfg.Source
+	cfg.Port = e.cfg.Port
+	cfg.StreamIndex = e.cfg.StreamIndex
+
+	e.cfg = cfg
+	return nil
+}
+
+// getCfg returns a snapshot of the current config.
+func (e *Engine) getCfg() Config {
+	e.cfgMu.RLock()
+	defer e.cfgMu.RUnlock()
+	return e.cfg
+}
+
 // New creates an engine from cfg. Capture is opened eagerly so that
 // misconfiguration (e.g. no screen-share permission) surfaces before Start.
 func New(cfg Config) (*Engine, error) {
@@ -274,9 +304,11 @@ func (e *Engine) audioLoop(src io.ReadCloser) {
 	}
 }
 
-// videoLoop pulls frames and sends them at the configured fps.
+// videoLoop pulls frames and sends them at the configured fps. The frame rate
+// and JPEG quality are read from the live config so SetConfig takes effect
+// without restarting.
 func (e *Engine) videoLoop() {
-	frameInterval := time.Second / time.Duration(e.cfg.FPS)
+	frameInterval := time.Second / time.Duration(e.getCfg().FPS)
 	next := e.start
 
 	for {
@@ -294,6 +326,15 @@ func (e *Engine) videoLoop() {
 			continue
 		}
 
+		// Re-read the live config each frame so FPS/quality changes apply
+		// immediately. When the interval changes, resync `next` to now.
+		cfg := e.getCfg()
+		interval := time.Second / time.Duration(cfg.FPS)
+		if interval != frameInterval {
+			frameInterval = interval
+			next = time.Now()
+		}
+
 		// Drop frames when running ahead of the target fps to keep
 		// timestamps monotonic (e.g. a 60 Hz monitor captured at 30 fps).
 		now := time.Now()
@@ -303,7 +344,7 @@ func (e *Engine) videoLoop() {
 		next = now.Add(frameInterval)
 
 		ts := uint64(now.Sub(e.start))
-		buf, err := e.encoder.EncodeBGRA(frame.Pix, frame.Width, frame.Height, e.cfg.Quality)
+		buf, err := e.encoder.EncodeBGRA(frame.Pix, frame.Width, frame.Height, cfg.Quality)
 		if err != nil {
 			e.setErr(err)
 			log.Printf("flinger: jpeg: %v", err)

internal/flinger/flinger_test.go

@@ -142,3 +142,50 @@ func itoa(v int) string {
 	}
 	return string(buf[i:])
 }
+
+// TestSetConfig verifies live config updates apply FPS/quality and reject
+// invalid values, while preserving fields that can't change live.
+func TestSetConfig(t *testing.T) {
+	cfg := DefaultConfig()
+	cfg.Source = "pattern"
+	cfg.Port = 19758
+
+	eng, err := New(cfg)
+	if err != nil {
+		t.Fatalf("New: %v", err)
+	}
+
+	// Update FPS/quality live.
+	newCfg := DefaultConfig()
+	newCfg.Source = "screen" // must be ignored (requires restart)
+	newCfg.Port = 9999       // must be ignored
+	newCfg.Quality = 95
+	newCfg.FPS = 60
+	if err := eng.SetConfig(newCfg); err != nil {
+		t.Fatalf("SetConfig: %v", err)
+	}
+
+	got := eng.getCfg()
+	if got.Quality != 95 {
+		t.Errorf("quality = %d, want 95", got.Quality)
+	}
+	if got.FPS != 60 {
+		t.Errorf("fps = %d, want 60", got.FPS)
+	}
+	// Source/port preserved.
+	if got.Source != "pattern" {
+		t.Errorf("source = %q, want pattern (unchanged live)", got.Source)
+	}
+	if got.Port != 19758 {
+		t.Errorf("port = %d, want 19758 (unchanged live)", got.Port)
+	}
+
+	// Invalid config is rejected and current settings kept.
+	if err := eng.SetConfig(Config{Quality: 200}); err == nil {
+		t.Error("expected error for invalid quality")
+	}
+	got = eng.getCfg()
+	if got.Quality != 95 {
+		t.Errorf("quality changed after rejected update: %d", got.Quality)
+	}
+}