feat : add admin page for task configuration

Add admin page with tabs for managing task statuses, efforts,
priorities and types, with CRUD drawers and color picker.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/layouts/default.vue

@@ -30,6 +30,14 @@
                         <Icon name="mdi:account-group-outline" size="24"/>
                         <span class="self-baseline text-md">Clients</span>
                     </NuxtLink>
+                    <NuxtLink
+                        to="/admin"
+                        class="flex gap-3 px-4 py-3 text-md font-semibold text-black hover:bg-tertiary-500 hover:text-primary-500"
+                        active-class="bg-tertiary-500 text-primary-500"
+                    >
+                        <Icon name="mdi:cog-outline" size="24"/>
+                        <span class="self-baseline text-md">Administration</span>
+                    </NuxtLink>
                 </nav>
 
                 <div class="flex flex-col gap-2 items-center p-4">