Refactor: Centralize template loading and improve error handling

- Introduced helpers.LoadTemplateFiles() for consistent layout + topbar rendering
- Replaced repeated template.ParseFiles() calls across handlers
- Created generic RenderError(w, r, statusCode) helper
- Replaced old Render403 with flexible RenderError
- Updated AdminOnly middleware to render 403 errors with context
- Added 500.html template for graceful panic fallback
- Prepared structure for future error codes (404, 429, etc.)

helpers/pages.go

@@ -2,18 +2,23 @@ package helpers
 
 // ToDo should be a handler?
 import (
-	"html/template"
+	"fmt"
+	"log"
 	"net/http"
 
 	"synlotto-website/models"
 )
 
-func Render403(w http.ResponseWriter, r *http.Request) {
+func RenderError(w http.ResponseWriter, r *http.Request, statusCode int) {
 	context := TemplateContext(w, r, models.TemplateData{})
-	tmpl := template.Must(template.New("").Funcs(TemplateFuncs()).ParseFiles(
-		"templates/layout.html",
-		"templates/error/403.html",
-	))
 
-	tmpl.ExecuteTemplate(w, "layout", context)
+	page := fmt.Sprintf("templates/error/%d.html", statusCode)
+	tmpl := LoadTemplateFiles(fmt.Sprintf("%d.html", statusCode), page)
+
+	w.WriteHeader(statusCode)
+	err := tmpl.ExecuteTemplate(w, "layout", context)
+	if err != nil {
+		log.Printf("❌ Failed to render error page for %d: %v", statusCode, err)
+		http.Error(w, http.StatusText(statusCode), statusCode)
+	}
 }