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

docs/ARCHITECTURE.md

@@ -86,3 +86,14 @@ A dedicated side panel provides visibility into the background queue system.
     - Displays pending edits with timestamps.
     - Provides controls to globally enable/disable processing.
     - Allows manual intervention (delete/push) for individual items.
+
+### 6. Service Layer & Testing
+
+To enable unit testing without Google Apps Script dependencies, the project uses a Service pattern with Dependency Injection.
+
+- **`ISpreadsheetService`**: Interface for all sheet interactions.
+- **`GASSpreadsheetService`**: Production implementation wrapping `SpreadsheetApp`.
+- **`MockSpreadsheetService`**: In-memory implementation for tests.
+
+Classes (like `Product`) should accept an `ISpreadsheetService` in their constructor. This allows providing the Mock service during tests to verify logic without touching real Google Sheets.
+