Add invite button and InviteModal to task detail page

Adds the 'Pozvat' button to the collaboration section of the task
detail page, opening a modal that creates invitations and shows
WhatsApp/Telegram/SMS/Copy share links.

apps/tasks/app/tasks/[id]/page.tsx

@@ -329,6 +329,15 @@ export default function TaskDetailPage() {
             </svg>
             {t("collab.collaboration")}
           </button>
+          <button
+            onClick={() => setShowInvite(true)}
+            className="inline-flex items-center gap-1.5 px-3 py-1.5 text-sm font-medium bg-blue-600 hover:bg-blue-700 text-white rounded-lg transition-colors"
+          >
+            <svg className="w-4 h-4" fill="none" viewBox="0 0 24 24" stroke="currentColor" strokeWidth={2}>
+              <path strokeLinecap="round" strokeLinejoin="round" d="M12 4.354a4 4 0 110 5.292M15 21H3v-1a6 6 0 0112 0v1zm0 0h6v-1a6 6 0 00-9-5.197M13 7a4 4 0 11-8 0 4 4 0 018 0z" />
+            </svg>
+            Pozvat
+          </button>
         </div>
 
         {/* Assigned users */}