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/SETUP.md

@@ -2,23 +2,24 @@
 
 ## Local Development Environment
 
-1.  **Install Node.js**
-    This project uses `fnm` to manage Node versions.
+1.  **Environment Initialization**
+    Run the provided PowerShell script to automatically install:
+    - `fnm` (Fast Node Manager) via Winget
+    - Node.js (v22)
+    - Global dependencies (`@google/clasp`)
+    - Project dependencies (`npm install`)
+
     ```powershell
-    # Install fnm (if not installed)
-    winget install Schniz.fnm
-    # Configure environment
-    fnm env --use-on-cd | Out-String | Invoke-Expression
-    # Install Node version
-    fnm use --install-if-missing 22
+    .\init.ps1
     ```
 
-2.  **Install Dependencies**
+2.  **Verify Installation**
+    Run tests to confirm the environment is correctly configured.
     ```bash
-    npm install
-    npm install -g @google/clasp
+    npm test
     ```
 
+
 3.  **Clasp Login**
     Authenticate with Google to allow pushing code.