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)
 }