feat: add Fyne desktop GUI with system tray (M3)

- internal/gui: settings form (name, port, source, quality, fps, audio)
  wired to the flinger engine, config save/load, start/stop toggle
- system tray via Fyne's StatusNotifierItem: Show (raises window),
  Start/Stop, Quit; left-click shows the settings window
- cmd/teleportfling-gui entry point

Verified on Hyprland: tray icon registers, start/stop streaming works,
window shows and focuses from the tray.

cmd/teleportfling-gui/main.go

@@ -0,0 +1,14 @@
+// Command teleportfling-gui is the desktop front-end for TeleportFling: a
+// Fyne settings window plus a system tray to start/stop streaming.
+//
+// The streaming engine lives in internal/flinger; this command only wires the
+// GUI and tray around it. Headless use is handled by cmd/teleportfling.
+package main
+
+import (
+	"teleportfling/internal/gui"
+)
+
+func main() {
+	gui.Run()
+}