[client/ui] Replace fyne UI with Wails (rename ui-wails to ui)

Removes the legacy fyne-based client/ui implementation and renames the
Wails replacement (client/ui-wails) to take its place at client/ui. Go
imports, frontend bindings, CI workflows, goreleaser configs and the
windows .syso icon path are updated to follow the rename.

client/ui/tray_linux.go

@@ -0,0 +1,24 @@
+//go:build linux && !386
+
+package main
+
+import "os"
+
+// init runs before Wails' own init(), so the env var is set in time.
+func init() {
+	if os.Getenv("WEBKIT_DISABLE_DMABUF_RENDERER") != "" {
+		return
+	}
+
+	// WebKitGTK's DMA-BUF renderer fails on many setups (VMs, containers,
+	// minimal WMs without proper GPU access) and leaves the window blank
+	// white. Wails only disables it for NVIDIA+Wayland, but the issue is
+	// broader. Always disable it — software rendering works fine for a
+	// small UI like this.
+	_ = os.Setenv("WEBKIT_DISABLE_DMABUF_RENDERER", "1")
+}
+
+// On Linux, the system tray provider may require the menu to be recreated
+// rather than updated in place. The rebuildExitNodeMenu method in tray.go
+// already handles this by removing and re-adding items; no additional
+// Linux-specific workaround is needed for Wails v3.