feat(avatar) : add UserAvatar component with image/initials fallback

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

frontend/components/user/UserAvatar.vue

@@ -0,0 +1,55 @@
+<template>
+    <span
+        class="inline-flex shrink-0 items-center justify-center rounded-full"
+        :class="sizeClasses"
+        :title="user.username"
+    >
+        <img
+            v-if="user.avatarUrl && !imgError"
+            :src="user.avatarUrl"
+            :alt="user.username"
+            class="h-full w-full rounded-full object-cover"
+            @error="imgError = true"
+        />
+        <span
+            v-else
+            class="flex h-full w-full items-center justify-center rounded-full bg-primary-500 font-bold text-white"
+            :class="textSizeClass"
+        >
+            {{ user.username.substring(0, 2).toUpperCase() }}
+        </span>
+    </span>
+</template>
+
+<script setup lang="ts">
+const props = defineProps<{
+    user: { id?: number; username: string; avatarUrl?: string | null }
+    size?: 'xs' | 'sm' | 'md' | 'lg'
+}>()
+
+const imgError = ref(false)
+
+watch(() => props.user.avatarUrl, () => {
+    imgError.value = false
+})
+
+const sizeClasses = computed(() => {
+    const map = {
+        xs: 'h-5 w-5',
+        sm: 'h-6 w-6',
+        md: 'h-8 w-8',
+        lg: 'h-12 w-12',
+    }
+    return map[props.size ?? 'sm']
+})
+
+const textSizeClass = computed(() => {
+    const map = {
+        xs: 'text-[10px]',
+        sm: 'text-xs',
+        md: 'text-sm',
+        lg: 'text-base',
+    }
+    return map[props.size ?? 'sm']
+})
+</script>