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/server/server.go

@@ -165,6 +165,8 @@ func (s *Server) ListenAndServe(ctx context.Context) error {
 	mux.HandleFunc("GET /api/presign/chapter/{slug}/{n}", s.handlePresignChapter)
 	mux.HandleFunc("GET /api/presign/audio/{slug}/{n}", s.handlePresignAudio)
 	mux.HandleFunc("GET /api/presign/voice-sample/{voice}", s.handlePresignVoiceSample)
+	mux.HandleFunc("GET /api/presign/avatar-upload/{userId}", s.handlePresignAvatarUpload)
+	mux.HandleFunc("GET /api/presign/avatar/{userId}", s.handlePresignAvatar)
 	// Plain-text chapter content (used server-side for audio generation)
 	mux.HandleFunc("GET /api/chapter-text/{slug}/{n}", s.handleChapterText)
 	// Voices list (proxied from Kokoro)