feat: support custom --config path in CLI and GUI

Add config.LoadFrom/SaveTo for arbitrary paths and thread a --config
flag through both entry points:
- CLI: --config loads a file first; explicit flags override file values
- GUI: --config selects the settings file used for load and save
- tests for LoadFrom/SaveTo and default fallback

cmd/teleportfling-gui/main.go

@@ -3,12 +3,21 @@
 //
 // The streaming engine lives in internal/flinger; this command only wires the
 // GUI and tray around it. Headless use is handled by cmd/teleportfling.
+//
+// Usage:
+//
+//	teleportfling-gui [--config PATH]
 package main
 
 import (
+	"flag"
+
 	"teleportfling/internal/gui"
 )
 
 func main() {
-	gui.Run()
+	configPath := flag.String("config", "", "config file path (default: ~/.config/teleportfling/config.json)")
+	flag.Parse()
+
+	gui.Run(*configPath)
 }

cmd/teleportfling/main.go

@@ -14,10 +14,15 @@
 //	teleportfling [--name NAME] [--port PORT] [--quality 1..100]
 //	             [--fps N] [--source screen|pattern] [--audio]
 //	             [--stream-index N] [--duration SECONDS]
+//	             [--config PATH]
 //
 // --source pattern selects the M1 synthetic test pattern (colour bars with a
 // moving box) instead of real screen capture, which is useful for testing
 // without granting screen-share permission.
+//
+// --config loads a saved config file first; any flag given explicitly on the
+// command line overrides the file value. When running under a service manager
+// (e.g. a systemd user unit) use --config to point at the daemon's profile.
 package main
 
 import (
@@ -28,6 +33,7 @@ import (
 	"syscall"
 	"time"
 
+	"teleportfling/internal/config"
 	"teleportfling/internal/flinger"
 )
 
@@ -42,19 +48,33 @@ func main() {
 		noAnnounce  = flag.Bool("no-announce", false, "do not announce on the LAN (receiver must connect by IP)")
 		streamIndex = flag.Int("stream-index", 0, "monitor index to capture (screen source)")
 		duration    = flag.Duration("duration", 0, "stream duration (0 = run until interrupted)")
+		configPath  = flag.String("config", "", "config file path (default: ~/.config/teleportfling/config.json)")
 	)
 	flag.Parse()
 
-	cfg := flinger.Config{
-		Name:        *name,
-		Port:        *port,
-		Quality:     *quality,
-		FPS:         *fps,
-		Source:      *source,
-		Audio:       *withAudio,
-		StreamIndex: *streamIndex,
-		Announce:    !*noAnnounce,
-	}
+	// Base config: loaded from file (or defaults when absent), then overridden
+	// by any flag the user explicitly set.
+	cfg := loadCLIConfig(*configPath)
+	flag.Visit(func(f *flag.Flag) {
+		switch f.Name {
+		case "name":
+			cfg.Name = *name
+		case "port":
+			cfg.Port = *port
+		case "quality":
+			cfg.Quality = *quality
+		case "fps":
+			cfg.FPS = *fps
+		case "source":
+			cfg.Source = *source
+		case "audio":
+			cfg.Audio = *withAudio
+		case "no-announce":
+			cfg.Announce = !*noAnnounce
+		case "stream-index":
+			cfg.StreamIndex = *streamIndex
+		}
+	})
 
 	eng, err := flinger.New(cfg)
 	if err != nil {
@@ -85,3 +105,19 @@ func main() {
 	eng.Stop()
 	log.Printf("teleportfling: stopped")
 }
+
+// loadCLIConfig returns the base flinger config. With a --config path it reads
+// that file; otherwise it reads the default user config. Missing files fall
+// back to defaults.
+func loadCLIConfig(path string) flinger.Config {
+	p := path
+	if p == "" {
+		p = config.Path()
+	}
+	c, err := config.LoadFrom(p)
+	if err != nil {
+		log.Printf("teleportfling: config %s: %v (using defaults)", p, err)
+		return flinger.DefaultConfig()
+	}
+	return c.ToFlinger()
+}