docs: add claude code documentation

- Added CLAUDE.md with project overview and architecture
- Documented development commands for PHP and frontend workflows
- Included code structure, configuration, and quality tool information

Author: sectsect

CLAUDE.md

@@ -0,0 +1,94 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+Google Spreadsheet to DB is a WordPress plugin that imports data from Google Sheets into WordPress database using Google's Sheets API (v4). This is a PHP/TypeScript hybrid project with a modern frontend build system.
+
+## Development Commands
+
+### PHP Development
+```bash
+# Install PHP dependencies
+cd functions/composer/
+composer install
+
+# Run PHPStan static analysis
+composer phpstan
+
+# Run PHPUnit tests
+./vendor/bin/phpunit
+
+# Run PHP CodeSniffer
+./vendor/bin/phpcs
+
+# Clear PHPStan cache
+composer phpstan-clear
+```
+
+### Frontend Development
+```bash
+# Install Node.js dependencies
+pnpm install
+
+# Development build with watch mode
+pnpm dev
+
+# Production build
+pnpm build
+
+# Type checking
+pnpm type-check
+pnpm type-check:watch
+
+# Linting
+pnpm lint
+pnpm lint:fix
+pnpm lint:css
+```
+
+## Code Architecture
+
+### PHP Backend Structure
+- **Main Plugin File**: `google-spreadsheet-to-db.php` - Entry point, handles plugin activation and includes
+- **Core Classes**: Located in `includes/` directory
+  - `class-google-spreadsheet-to-db-query.php` - Database query interface for retrieving stored data
+  - `class-google-spreadsheet-to-db-activator.php` - Plugin activation logic
+  - `admin.php` - WordPress admin interface
+  - `save.php` - Data saving functionality
+  - `index.php` - Core plugin functionality
+- **Helper Functions**: `functions/functions.php`
+- **Database**: Uses custom table `wp_google_ss2db` to store spreadsheet data as JSON
+
+### Frontend Structure
+- **TypeScript Source**: `src/assets/ts/` - Modular TypeScript components
+- **CSS Source**: `src/assets/css/` - PostCSS stylesheets
+- **Build Tool**: Rspack (configured in `rspack.config.ts`)
+
+### Configuration Constants
+Plugin requires WordPress constants in `wp-config.php`:
+- `GOOGLE_SS2DB_CLIENT_SECRET_PATH` - Path to Google API credentials JSON file
+- `GOOGLE_SS2DB_DEBUG` (optional) - Enable debug mode for detailed JSON responses
+
+## Key Features
+- Imports Google Sheets data via API and stores as JSON in WordPress database
+- Provides WordPress admin interface for configuration
+- Offers hooks for data manipulation before/after save (`google_ss2db_before_save`, `google_ss2db_after_save`)
+- Query API for retrieving stored data with filtering and sorting capabilities
+
+## Quality Tools
+- **PHP**: PHPStan (level 9), PHPCS with WordPress Coding Standards, PHPUnit
+- **TypeScript**: ESLint with Airbnb config, TypeScript strict mode
+- **CSS**: Stylelint with standard config
+- **Formatting**: Prettier for JS/TS/CSS
+
+## Testing
+- PHP tests located in `tests/` directory
+- Uses PHPUnit with WordPress test framework
+- Test configuration in `phpunit.xml.dist`
+
+## Dependencies
+- PHP 8.0+ required
+- Google API Client Library for PHP
+- Modern Node.js toolchain (pnpm, TypeScript, Rspack)