[client/uiwails] Windows cross-build support and notification actions

- Add build/windows/Taskfile.yml to cross-compile from Linux with
  mingw-w64 (CC=x86_64-w64-mingw32-gcc, CGO_ENABLED=1, -H=windowsgui).
- Rename xembed_tray.{c,h} to xembed_tray_linux.{c,h} so the Go
  toolchain only compiles these X11/GTK sources on Linux.
- Add sendShowWindowSignal on Windows: opens the named Global event
  and calls SetEvent so the already-running instance shows its window.
- Register a notification category with Open/Dismiss action buttons
  and wire a response handler. Do this inside the ApplicationStarted
  hook so it runs after the notifications service's Startup has
  initialized appName/appGUID on Windows; otherwise the category is
  saved under an empty registry path and SendNotificationWithActions
  silently falls back to a plain toast with no buttons.
- Bump github.com/wailsapp/wails/v3 to v3.0.0-alpha.78.

client/uiwails/Taskfile.yml

@@ -4,6 +4,7 @@ includes:
   common: ./build/Taskfile.yml
   linux: ./build/linux/Taskfile.yml
   darwin: ./build/darwin/Taskfile.yml
+  windows: ./build/windows/Taskfile.yml
 
 vars:
   APP_NAME: "netbird-ui"