Ship a legacy GTK3 NetBird UI package alongside the GTK4 one

Wails v3 builds on GTK4/WebKitGTK 6.0 by default, but distros that don't
ship WebKitGTK 6.0 yet (Ubuntu 22.04, Debian 12, RHEL 9, Fedora <=39)
can't install the GTK4 packages. Add a parallel legacy build via the
-tags gtk3 path (removed upstream in Wails v3.1).

The project's own XEmbed tray host links GTK4 and uses GTK4-only
popup-menu APIs, so it can't compile against GTK3. Rather than port the
~150-line C menu layer, exclude it on gtk3 builds (!gtk3 constraint on
xembed_host_linux.go + xembed_tray_linux.c) and add a pure-Go stub
(xembed_host_gtk3_linux.go) where xembedTrayAvailable() returns false.
The watcher probe then exits immediately, so the in-process XEmbed
fallback is absent on gtk3 builds -- the tray still works on desktops
that ship their own StatusNotifierWatcher, only the minimal-WM fallback
is unavailable.

goreleaser gains a netbird-ui-gtk3 build plus deb/rpm nfpm blocks (same
netbird-ui package name, GTK3/WebKit2GTK 4.1 deps) routed to dedicated
legacy repo paths. The release_ui CI job installs both dev stacks on the
runner so goreleaser builds both Linux variants natively.

.github/workflows/release.yml

@@ -367,7 +367,10 @@ jobs:
           version: 11
 
       - name: Install dependencies
-        run: sudo apt update && sudo apt install -y -q libgtk-4-dev libwebkitgtk-6.0-dev libsoup-3.0-dev gcc-mingw-w64-x86-64
+        # GTK4/WebKitGTK 6.0 dev libs for the default build + GTK3/WebKit2GTK 4.1
+        # dev libs for the legacy -tags gtk3 build (netbird-ui-gtk3). Both stacks
+        # coexist on the same runner; goreleaser builds both Linux variants here.
+        run: sudo apt update && sudo apt install -y -q libgtk-4-dev libwebkitgtk-6.0-dev libsoup-3.0-dev libgtk-3-dev libwebkit2gtk-4.1-dev gcc-mingw-w64-x86-64
 
       - name: Decode GPG signing key
         if: github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name == github.repository