[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.

go.mod

@@ -101,6 +101,7 @@ require (
 	github.com/ti-mo/conntrack v0.5.1
 	github.com/ti-mo/netfilter v0.5.2
 	github.com/vmihailenco/msgpack/v5 v5.4.1
+	github.com/wailsapp/wails/v3 v3.0.0-alpha.78
 	github.com/yusufpapurcu/wmi v1.2.4
 	github.com/zcalusic/sysinfo v1.1.3
 	go.opentelemetry.io/contrib/instrumentation/google.golang.org/grpc/otelgrpc v0.61.0
@@ -134,6 +135,7 @@ require (
 	cloud.google.com/go/compute/metadata v0.9.0 // indirect
 	dario.cat/mergo v1.0.2 // indirect
 	filippo.io/edwards25519 v1.1.0 // indirect
+	git.sr.ht/~jackmordaunt/go-toast/v2 v2.0.3 // indirect
 	github.com/AppsFlyer/go-sundheit v0.6.0 // indirect
 	github.com/Azure/go-ansiterm v0.0.0-20230124172434-306776ec8161 // indirect
 	github.com/Azure/go-ntlmssp v0.0.0-20221128193559-754e69321358 // indirect
@@ -257,7 +259,6 @@ require (
 	github.com/nfnt/resize v0.0.0-20180221191011-83c6a9932646 // indirect
 	github.com/nicksnyder/go-i18n/v2 v2.5.1 // indirect
 	github.com/nxadm/tail v1.4.8 // indirect
-	github.com/onsi/ginkgo/v2 v2.19.0 // indirect
 	github.com/opencontainers/go-digest v1.0.0 // indirect
 	github.com/opencontainers/image-spec v1.1.1 // indirect
 	github.com/pion/dtls/v2 v2.2.10 // indirect
@@ -291,7 +292,6 @@ require (
 	github.com/vishvananda/netns v0.0.5 // indirect
 	github.com/vmihailenco/tagparser/v2 v2.0.0 // indirect
 	github.com/wailsapp/go-webview2 v1.0.23 // indirect
-	github.com/wailsapp/wails/v3 v3.0.0-alpha.74 // indirect
 	github.com/wlynxg/anet v0.0.5 // indirect
 	github.com/xanzy/ssh-agent v0.3.3 // indirect
 	github.com/yuin/goldmark v1.7.16 // indirect