Fix embedded frontend serving: use fs.ReadFile instead of Gin FileFromFS

- FileFromFS with http.FS(embedFS) returned 301 redirects due to
  Gin setting c.Request.URL.Path to a relative path for http.FileServer
- Replace with direct fs.ReadFile + Gin c.Data, determining content
  type via mime.TypeByExtension
- Supports all asset types (JS, CSS, SVG, HTML) and SPA fallback

backend/cmd/server/main.go

@@ -4,10 +4,12 @@ import (
 	"context"
 	"io/fs"
 	"log"
+	"mime"
 	"net/http"
 	"os"
 	"os/signal"
 	"path/filepath"
+	"strings"
 	"syscall"
 	"time"
 
@@ -59,10 +61,22 @@ func main() {
 	subFS, subErr := fs.Sub(embed.Frontend, "dist")
 	if subErr == nil {
 		if _, err := subFS.Open("index.html"); err == nil {
-			r.StaticFS("/assets", http.FS(subFS))
-			r.StaticFileFS("/favicon.ico", "favicon.ico", http.FS(subFS))
 			r.NoRoute(func(c *gin.Context) {
-				c.FileFromFS("index.html", http.FS(subFS))
+				path := strings.TrimPrefix(c.Request.URL.Path, "/")
+				if path == "" {
+					path = "index.html"
+				}
+				data, err := fs.ReadFile(subFS, path)
+				if err == nil {
+					ct := mime.TypeByExtension(filepath.Ext(path))
+					if ct == "" {
+						ct = "text/html; charset=utf-8"
+					}
+					c.Data(http.StatusOK, ct, data)
+					return
+				}
+				data, _ = fs.ReadFile(subFS, "index.html")
+				c.Data(http.StatusOK, "text/html; charset=utf-8", data)
 			})
 			log.Print("serving embedded frontend")
 		}