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/orchestrator/orchestrator_test.go

@@ -151,6 +151,13 @@ func (s *mockStore) PresignChapter(_ context.Context, _ string, _ int, _ time.Du
 func (s *mockStore) PresignAudio(_ context.Context, _ string, _ time.Duration) (string, error) {
 	return "", nil
 }
+func (s *mockStore) PresignAvatarUpload(_ context.Context, _, _ string) (string, string, error) {
+	return "", "", nil
+}
+func (s *mockStore) PresignAvatarURL(_ context.Context, _ string) (string, bool, error) {
+	return "", false, nil
+}
+func (s *mockStore) DeleteAvatar(_ context.Context, _ string) error      { return nil }
 func (s *mockStore) SaveBrowsePage(_ context.Context, _, _ string) error { return nil }
 func (s *mockStore) GetBrowsePage(_ context.Context, _ string) (string, bool, error) {
 	return "", false, nil