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/config/config.go

@@ -71,15 +71,20 @@ var pathVar = func() string {
 	return filepath.Join(dir, "teleportfling", "config.json")
 }()
 
-// Path returns the config file location.
+// Path returns the default config file location.
 func Path() string {
 	return pathVar
 }
 
-// Load reads the config file, returning Default when it does not exist.
+// Load reads the default config file, returning Default when it does not exist.
 func Load() (Config, error) {
-	p := Path()
-	data, err := os.ReadFile(p)
+	return LoadFrom(Path())
+}
+
+// LoadFrom reads the config file at path, returning Default when it does not
+// exist. This lets the CLI and GUI support custom --config paths.
+func LoadFrom(path string) (Config, error) {
+	data, err := os.ReadFile(path)
 	if err != nil {
 		if errors.Is(err, os.ErrNotExist) {
 			return Default(), nil
@@ -113,15 +118,19 @@ func Load() (Config, error) {
 	return c, nil
 }
 
-// Save writes the config file, creating the directory if needed.
+// Save writes the default config file, creating the directory if needed.
 func Save(c Config) error {
-	p := Path()
-	if err := os.MkdirAll(filepath.Dir(p), 0o755); err != nil {
+	return SaveTo(Path(), c)
+}
+
+// SaveTo writes the config file at path, creating the directory if needed.
+func SaveTo(path string, c Config) error {
+	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
 		return err
 	}
 	data, err := json.MarshalIndent(c, "", "  ")
 	if err != nil {
 		return err
 	}
-	return os.WriteFile(p, data, 0o600)
+	return os.WriteFile(path, data, 0o600)
 }