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/home.go

@@ -2,7 +2,6 @@ package handlers
 
 import (
 	"database/sql"
-	"html/template"
 	"log"
 	"net/http"
 	"synlotto-website/helpers"
@@ -13,11 +12,7 @@ func Home(db *sql.DB) http.HandlerFunc {
 		data := BuildTemplateData(db, w, r)
 		context := helpers.TemplateContext(w, r, data)
 
-		tmpl := template.Must(template.New("index.html").Funcs(helpers.TemplateFuncs()).ParseFiles(
-			"templates/layout.html",
-			"templates/topbar.html",
-			"templates/index.html",
-		))
+		tmpl := helpers.LoadTemplateFiles("index.html", "templates/index.html")
 
 		err := tmpl.ExecuteTemplate(w, "layout", context)
 		if err != nil {