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/notifications.go

@@ -5,7 +5,6 @@ import (
 	"log"
 	"net/http"
 	"strconv"
-	"text/template"
 
 	"synlotto-website/helpers"
 	"synlotto-website/storage"
@@ -16,13 +15,7 @@ func NotificationsHandler(db *sql.DB) http.HandlerFunc {
 		data := BuildTemplateData(db, w, r)
 		context := helpers.TemplateContext(w, r, data)
 
-		tmpl := template.Must(template.New("notifications.html").
-			Funcs(helpers.TemplateFuncs()).
-			ParseFiles(
-				"templates/layout.html",
-				"templates/topbar.html",
-				"templates/account/notifications.html",
-			))
+		tmpl := helpers.LoadTemplateFiles("notifications.html", "templates/account/notifications.html")
 
 		err := tmpl.ExecuteTemplate(w, "layout", context)
 		if err != nil {
@@ -63,13 +56,7 @@ func MarkNotificationReadHandler(db *sql.DB) http.HandlerFunc {
 		context := helpers.TemplateContext(w, r, data)
 		context["Notification"] = notification
 
-		tmpl := template.Must(template.New("read.html").
-			Funcs(helpers.TemplateFuncs()).
-			ParseFiles(
-				"templates/layout.html",
-				"templates/topbar.html",
-				"templates/account/notifications/read.html",
-			))
+		tmpl := helpers.LoadTemplateFiles("read.html", "templates/account/notifications/read.html")
 
 		err = tmpl.ExecuteTemplate(w, "layout", context)
 		if err != nil {