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/template.go

@@ -32,7 +32,6 @@ func TemplateContext(w http.ResponseWriter, r *http.Request, data models.Templat
 	}
 }
 
-// TemplateFuncs provides helper functions to be used in templates.
 func TemplateFuncs() template.FuncMap {
 	return template.FuncMap{
 		"plus1": func(i int) int { return i + 1 },
@@ -63,7 +62,16 @@ func TemplateFuncs() template.FuncMap {
 	}
 }
 
-// SetFlash sets a flash message in session.
+func LoadTemplateFiles(name string, files ...string) *template.Template {
+	shared := []string{
+		"templates/layout.html",
+		"templates/topbar.html",
+	}
+	all := append(shared, files...)
+
+	return template.Must(template.New(name).Funcs(TemplateFuncs()).ParseFiles(all...))
+}
+
 func SetFlash(w http.ResponseWriter, r *http.Request, message string) {
 	session, _ := GetSession(w, r)
 	session.Values["flash"] = message