feat: add diagnostic web app shell

internal/webapp/assets.go

@@ -0,0 +1,41 @@
+package webapp
+
+import (
+	"embed"
+	"io/fs"
+	"net/http"
+	"strings"
+)
+
+//go:embed static/*
+var assets embed.FS
+
+func Handler() http.Handler {
+	static, err := fs.Sub(assets, "static")
+	if err != nil {
+		panic(err)
+	}
+	files := http.FileServer(http.FS(static))
+
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path == "/" {
+			serveIndex(w, r, static)
+			return
+		}
+		if strings.HasPrefix(r.URL.Path, "/assets/") {
+			http.StripPrefix("/assets/", files).ServeHTTP(w, r)
+			return
+		}
+		http.NotFound(w, r)
+	})
+}
+
+func serveIndex(w http.ResponseWriter, r *http.Request, static fs.FS) {
+	content, err := fs.ReadFile(static, "index.html")
+	if err != nil {
+		http.Error(w, "web app unavailable", http.StatusInternalServerError)
+		return
+	}
+	w.Header().Set("Content-Type", "text/html; charset=utf-8")
+	_, _ = w.Write(content)
+}