Implement ReceiptStorageRepository for Supabase Storage

The Supabase Dart client's storage upload does not natively stream progress — implement progress by splitting large files into chunks using Uint8List.sublist() and uploading via multipart, or use the uploadBinary method with a custom HttpClient wrapper that intercepts sent bytes. A simpler approach for the initial implementation: use the standard upload call and emit progress as 0.0 (start) and 1.0 (complete) only, with a TODO comment for streaming progress in a future iteration. For MIME type magic byte detection: JPEG starts with bytes [0xFF, 0xD8, 0xFF]; PNG starts with [0x89, 0x50, 0x4E, 0x47]. Implement a private _detectMimeType(Uint8List bytes) method checking the first 4 bytes.

For retry logic, implement a generic _withRetry(Future Function() operation, {int maxAttempts = 3}) helper using Future.delayed with exponential backoff. Store bucket name and max expiry as constants rather than magic strings.

Write unit tests with a MockSupabaseStorageClient using mocktail: (1) uploadReceiptImage triggers onProgress callbacks at expected intervals and returns the storage path, (2) uploadReceiptImage throws ValidationException for files > 10MB without making a network call, (3) uploadReceiptImage throws ValidationException for non-image MIME types, (4) getSignedUrl returns a URL string on success and throws ReceiptStorageException on storage 404, (5) deleteReceiptImage does not throw when the file is absent (idempotent), (6) retry logic: mock the upload to throw SocketException twice then succeed on third attempt; assert the method returns successfully and the progress callback was called. Write one integration test against a local Supabase storage instance verifying end-to-end upload + signed URL retrieval.