feat: single Docker image with frontend served from backend

- Dockerfile: 3-stage build (frontend npm build → backend go build → alpine runtime)
- Frontend dist files served from disk at /app/frontend/dist
- server.go: catch-all GET handler serves static files with SPA fallback to index.html
- config.go: new server.frontend_dir option (env: SUB_STORE_SERVER_FRONTEND_DIR)
- API routes (/api/*), download routes, and /health take priority over static serving

cmd/server.go

@@ -129,6 +129,14 @@ func startFiber(cfg *config.Config, db *sqlx.DB) error {
 		return c.JSON(fiber.Map{"status": "ok"})
 	})
 
+	// Serve frontend static files with SPA fallback.
+	// Registered after API/download routes so those take priority.
+	frontendDir := cfg.Server.FrontendDir
+	if frontendDir == "" {
+		frontendDir = "./frontend/dist"
+	}
+	registerStaticFrontend(app, frontendDir)
+
 	// Start server
 	addr := fmt.Sprintf("%s:%d", cfg.Server.Host, cfg.Server.Port)
 	logrus.Infof("Starting %s on %s", cfg.App.Name, addr)
@@ -159,3 +167,33 @@ func startFiber(cfg *config.Config, db *sqlx.DB) error {
 	logrus.Info("Server stopped")
 	return nil
 }
+
+// registerStaticFrontend serves static files from the given directory with
+// SPA fallback: any GET request that doesn't match a real file returns
+// index.html (so client-side routing works on fresh page loads).
+func registerStaticFrontend(app *fiber.App, dir string) {
+	if _, err := os.Stat(dir); err != nil {
+		logrus.WithError(err).Warn("frontend directory not found, static serving disabled")
+		return
+	}
+
+	indexFile := filepath.Join(dir, "index.html")
+
+	// Catch-all GET handler: try to serve the requested file from the dist
+	// directory; if the file doesn't exist, fall back to index.html (SPA).
+	app.Use(func(c fiber.Ctx) error {
+		if c.Method() != "GET" {
+			return c.Next()
+		}
+		path := c.Path()
+
+		// Try the exact file in the dist directory
+		filePath := filepath.Join(dir, filepath.Clean(path))
+		if info, err := os.Stat(filePath); err == nil && !info.IsDir() {
+			return c.SendFile(filePath)
+		}
+
+		// SPA fallback → index.html
+		return c.SendFile(indexFile)
+	})
+}