feat(avatar) : replace initials with UserAvatar component everywhere

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/components/task/TaskCard.vue

@@ -44,13 +44,12 @@
             >
                 {{ tag.label }}
             </span>
-            <span
+            <UserAvatar
                 v-if="task.assignee"
-                class="ml-auto flex h-5 w-5 items-center justify-center rounded-full bg-primary-500 text-[10px] font-bold text-white"
-                :title="task.assignee.username"
-            >
-                {{ task.assignee.username.substring(0, 2).toUpperCase() }}
-            </span>
+                :user="task.assignee"
+                size="xs"
+                class="ml-auto"
+            />
             <span
                 v-else
                 class="ml-auto flex h-5 w-5 items-center justify-center rounded-full bg-neutral-200 text-neutral-400"