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.)

handlers/account.go

@@ -1,7 +1,6 @@
 package handlers
 
 import (
-	"html/template"
 	"log"
 	"net/http"
 	"synlotto-website/helpers"
@@ -18,12 +17,7 @@ func Login(w http.ResponseWriter, r *http.Request) {
 			http.Redirect(w, r, "/", http.StatusSeeOther)
 			return
 		}
-
-		tmpl := template.Must(template.New("login.html").Funcs(helpers.TemplateFuncs()).ParseFiles(
-			"templates/layout.html",
-			"templates/topbar.html",
-			"templates/account/login.html",
-		))
+		tmpl := helpers.LoadTemplateFiles("login.html", "templates/account/login.html")
 
 		context := helpers.TemplateContext(w, r, models.TemplateData{})
 		context["csrfField"] = csrf.TemplateField(r)
@@ -101,10 +95,7 @@ func Logout(w http.ResponseWriter, r *http.Request) {
 
 func Signup(w http.ResponseWriter, r *http.Request) {
 	if r.Method == http.MethodGet {
-		tmpl := template.Must(template.ParseFiles(
-			"templates/layout.html",
-			"templates/account/signup.html",
-		))
+		tmpl := helpers.LoadTemplateFiles("signup.html", "templates/account/signup.html")
 
 		tmpl.ExecuteTemplate(w, "layout", map[string]interface{}{
 			"csrfField": csrf.TemplateField(r),