feat: add emoji reaction support for files

- Implement `ReactionService` to manage file reactions in the database.
- Add `POST /d/{boxID}/f/{fileID}/react` endpoint to handle user reactions.
- Add `GET /emoji/{pack}/{file}` endpoint to serve custom emoji assets.
- Support loading custom emoji packs dynamically from the data directory.
- Update README with instructions on configuring emoji reaction packs.

backend/libs/services/upload.go

@@ -137,6 +137,9 @@ func NewUploadService(maxUploadSize int64, dataDir, baseURL string, logger *slog
 	if err := os.MkdirAll(dbDir, 0o755); err != nil {
 		return nil, err
 	}
+	if err := os.MkdirAll(filepath.Join(dataDir, "emoji"), 0o755); err != nil {
+		return nil, err
+	}
 
 	db, err := bbolt.Open(filepath.Join(dbDir, "warpbox.bbolt"), 0o600, &bbolt.Options{Timeout: time.Second})
 	if err != nil {
@@ -957,6 +960,10 @@ func randomID(byteCount int) string {
 	return base64.RawURLEncoding.EncodeToString(data)
 }
 
+func RandomPublicToken(byteCount int) string {
+	return randomID(byteCount)
+}
+
 func hashPassword(password string) (string, string) {
 	salt := randomID(18)
 	return salt, passwordHash(salt, password)