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

internal/gui/gui.go

@@ -43,6 +43,9 @@ type App struct {
 	win     fyne.Window
 	desk    desktop.App
 
+	// configPath overrides the default config location ("" = default).
+	configPath string
+
 	cfg  config.Config
 	eng  *flinger.Engine
 	lock bool // serialises start/stop against UI actions
@@ -63,9 +66,10 @@ type App struct {
 	statsDone   chan struct{}
 }
 
-// Run starts the GUI and blocks until the app exits.
-func Run() {
-	g := &App{}
+// Run starts the GUI and blocks until the app exits. configPath selects a
+// non-default settings file ("" uses the default location).
+func Run(configPath string) {
+	g := &App{configPath: configPath}
 
 	g.fyneApp = app.NewWithID(appID)
 	g.win = g.fyneApp.NewWindow("TeleportFling")
@@ -74,7 +78,7 @@ func Run() {
 	g.iconActive = newTrayResource(color.NRGBA{R: 46, G: 125, B: 50, A: 255}, color.NRGBA{R: 150, G: 220, B: 140, A: 255})
 
 	// Load persisted settings (falling back to defaults).
-	g.cfg = mustLoadConfig()
+	g.cfg = g.mustLoadConfig()
 
 	g.buildUI()
 
@@ -89,9 +93,18 @@ func Run() {
 	g.win.ShowAndRun()
 }
 
-// mustLoadConfig loads the config, logging and falling back to defaults.
-func mustLoadConfig() config.Config {
-	c, err := config.Load()
+// mustLoadConfig loads the config (from the configured path, or the default),
+// logging and falling back to defaults on error.
+func (g *App) mustLoadConfig() config.Config {
+	var (
+		c   config.Config
+		err error
+	)
+	if g.configPath != "" {
+		c, err = config.LoadFrom(g.configPath)
+	} else {
+		c, err = config.Load()
+	}
 	if err != nil {
 		log.Printf("gui: config load: %v (using defaults)", err)
 		return config.Default()
@@ -201,7 +214,11 @@ func (g *App) start() {
 		StreamIndex: g.cfg.StreamIndex,
 		Announce:    &announce,
 	}
-	if err := config.Save(g.cfg); err != nil {
+	if g.configPath != "" {
+		if err := config.SaveTo(g.configPath, g.cfg); err != nil {
+			log.Printf("gui: config save: %v", err)
+		}
+	} else if err := config.Save(g.cfg); err != nil {
 		log.Printf("gui: config save: %v", err)
 	}