feat: add mandatory tag support to frontend and update ItemsPage to display mandatory tags

frontend/src/api.ts

@@ -1,6 +1,7 @@
 export interface Tag {
   id: string;
   name: string;
+  mandatory: boolean;
 }
 
 export interface Item {

frontend/src/pages/ItemsPage.tsx

@@ -151,11 +151,16 @@ export default function ItemsPage() {
                 "bg-gray-200 text-sm rounded px-1 py-0.5 flex items-center cursor-pointer " +
                 (newItemTags.includes(tag.id)
                   ? "bg-blue-200 text-blue-900 font-bold"
-                  : "")
+                  : "") +
+                (tag.mandatory ? " border-2 border-yellow-400" : "")
               }
               onClick={() => toggleNewItemTag(tag.id)}
+              title={tag.mandatory ? "Pflicht-Tag (mandatory)" : ""}
             >
               #{tag.name}
+              {tag.mandatory && (
+                <span className="ml-1 text-yellow-500 font-bold" title="Pflicht-Tag">*</span>
+              )}
             </span>
           ))}
           <button