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_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)
+	}
+}