app: add code for macOS and Windows apps under 'app' (#12933)

* app: add code for macOS and Windows apps under 'app'

* app: add readme

* app: windows and linux only for now

* ci: fix ui CI validation

---------

Co-authored-by: jmorganca <jmorganca@gmail.com>

app/ui/app.go

@@ -0,0 +1,44 @@
+//go:build windows || darwin
+
+package ui
+
+import (
+	"bytes"
+	"embed"
+	"errors"
+	"io/fs"
+	"net/http"
+	"strings"
+	"time"
+)
+
+//go:embed app/dist
+var appFS embed.FS
+
+// appHandler returns an HTTP handler that serves the React SPA.
+// It tries to serve real files first, then falls back to index.html for React Router.
+func (s *Server) appHandler() http.Handler {
+	// Strip the dist prefix so URLs look clean
+	fsys, _ := fs.Sub(appFS, "app/dist")
+	fileServer := http.FileServer(http.FS(fsys))
+
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		p := strings.TrimPrefix(r.URL.Path, "/")
+		if _, err := fsys.Open(p); err == nil {
+			// Serve the file directly
+			fileServer.ServeHTTP(w, r)
+			return
+		}
+		// Fallback – serve index.html for unknown paths so React Router works
+		data, err := fs.ReadFile(fsys, "index.html")
+		if err != nil {
+			if errors.Is(err, fs.ErrNotExist) {
+				http.NotFound(w, r)
+			} else {
+				http.Error(w, "Internal Server Error", http.StatusInternalServerError)
+			}
+			return
+		}
+		http.ServeContent(w, r, "index.html", time.Time{}, bytes.NewReader(data))
+	})
+}