feat: avatar upload via presigned PUT URL flow

- Go scraper: add PresignAvatarUploadURL/PresignAvatarURL/DeleteAvatar to
  Store interface, implement on HybridStore+MinioClient, register
  GET /api/presign/avatar-upload/{userId} and /api/presign/avatar/{userId}
- SvelteKit: replace direct AWS S3 SDK in minio.ts with presign calls to
  the Go scraper; rewrite avatar +server.ts (POST=presign, PATCH=record key)
- iOS: rewrite uploadAvatar() as 3-step presigned PUT flow; refactor
  chip components into shared ChipButton in CommonViews.swift

scraper/internal/storage/store.go

@@ -160,6 +160,17 @@ type Store interface {
 	// PresignAudio returns a presigned GET URL for an audio object.
 	PresignAudio(ctx context.Context, key string, expires time.Duration) (string, error)
 
+	// PresignAvatarUpload returns a short-lived presigned PUT URL for uploading
+	// an avatar image directly to MinIO, and the object key that will be stored.
+	// ext should be "jpg", "png", or "webp".
+	PresignAvatarUpload(ctx context.Context, userID, ext string) (uploadURL, key string, err error)
+
+	// PresignAvatarURL returns a presigned GET URL for a user's avatar, or ("", false, nil) if none.
+	PresignAvatarURL(ctx context.Context, userID string) (string, bool, error)
+
+	// DeleteAvatar removes all avatar objects for a user (all extensions).
+	DeleteAvatar(ctx context.Context, userID string) error
+
 	// ── Browse page snapshots (MinIO) ──────────────────────────────────────
 
 	// SaveBrowsePage stores a SingleFile HTML snapshot for the given cache key.