feat: add tray streaming status and app icon

- tray icon and tooltip now reflect engine state: grey when stopped,
  green + live frame/drop counters in the tooltip while streaming
- generate a proper app icon (assets/, via cmd/teleportfling-icon) and
  embed it so the window and tray carry the icon without runtime files
- add a .desktop entry template for launching from an app menu
- run Fyne UI updates (status label, tray icon/tooltip) on the main
  thread via fyne.Do to satisfy the threading model

assets/assets.go

@@ -0,0 +1,23 @@
+// Package assets embeds the TeleportFling application icon.
+//
+// The PNGs live in the repository assets/ directory and are generated by
+// cmd/teleportfling-icon. Embedding keeps the icon with the binary so the GUI
+// and tray never depend on a runtime file path.
+package assets
+
+import _ "embed"
+
+// AppIcon256 is the 256px application icon.
+//
+//go:embed teleportfling-256.png
+var AppIcon256 []byte
+
+// AppIcon64 is the 64px icon used by the system tray.
+//
+//go:embed teleportfling-64.png
+var AppIcon64 []byte
+
+// AppIcon is the full-resolution (512px) application icon.
+//
+//go:embed teleportfling.png
+var AppIcon []byte

assets/teleportfling.desktop

@@ -0,0 +1,11 @@
+[Desktop Entry]
+Type=Application
+Name=TeleportFling
+GenericName=Screen and audio stream sender
+Comment=Stream this screen and audio to an OBS Teleport receiver
+Exec=teleportfling-gui
+Icon=teleportfling
+Terminal=false
+Categories=AudioVideo;Video;Network;
+StartupNotify=false
+StartupWMClass=io.teleportfling