fix(avatar) : move avatar service to composables for Nuxt auto-import

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

frontend/composables/useAvatarService.ts

@@ -0,0 +1,24 @@
+export function useAvatarService() {
+    const api = useApi()
+
+    async function upload(userId: number, file: Blob): Promise<{ avatarUrl: string }> {
+        const formData = new FormData()
+        formData.append('file', file, 'avatar.png')
+
+        return $fetch(`/api/users/${userId}/avatar`, {
+            method: 'POST',
+            body: formData,
+            credentials: 'include',
+        })
+    }
+
+    async function remove(userId: number): Promise<void> {
+        await api.delete(`/users/${userId}/avatar`)
+    }
+
+    function getUrl(userId: number): string {
+        return `/api/users/${userId}/avatar`
+    }
+
+    return { upload, remove, getUrl }
+}