feat: introduce Jest testing framework and decouple Product logic

- Added Jest infrastructure (deps, config, global mocks)
- Introduced ISpreadsheetService with GAS and Mock implementations
- Refactored Product.ts to use dependency injection
- Added unit tests for Product class
- Updated documentation (README, SETUP, ARCHITECTURE) to reflect testing and init scripts

README.md

@@ -38,6 +38,13 @@ The system allows you to:
     npm run deploy
     ```
 
+## Testing
+
+Run unit tests using Jest:
+```bash
+npm test
+```
+
 ## Project Structure
 
 - `src/`: Source code (TypeScript)