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/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