feat: implement Phase 1 - Core Page Cache and Admin Settings UI

Implement the core page caching functionality and admin settings interface
for WP Recache plugin. This includes:

- Plugin bootstrap with activation/deactivation handlers
- Admin settings page with tabbed interface (Cache, Optimization, Advanced)
- Admin bar integration for quick cache purging
- Core page cache using output buffering
- Cache file management with mobile device separation
- Cache purge on post update/comment
- Default URL exclusions (cart, checkout, account)
- Cache statistics display
- PHPUnit test suite setup
- Docker-based development environment
- PHPStan configuration for static analysis

References:
- Closes #1 (Core Page Cache)
- Closes #8 (Admin Settings UI)

Dockerfile

@@ -0,0 +1,37 @@
+FROM php:8.1-cli
+
+# Install system dependencies
+RUN apt-get update && apt-get install -y \
+    git \
+    unzip \
+    libzip-dev \
+    libxml2-dev \
+    libicu-dev \
+    && docker-php-ext-install zip \
+    && docker-php-ext-install dom \
+    && docker-php-ext-install libxml \
+    && docker-php-ext-install intl \
+    && docker-php-ext-install mbstring \
+    && docker-php-ext-install pdo \
+    && docker-php-ext-install pdo_mysql
+
+# Install Composer
+COPY --from=composer:latest /usr/bin/composer /usr/bin/composer
+
+# Set working directory
+WORKDIR /app
+
+# Copy composer files first for caching
+COPY composer.json composer.lock* ./
+
+# Install dependencies
+RUN composer install --no-interaction --no-scripts --no-autoloader --prefer-dist
+
+# Copy application code
+COPY . .
+
+# Generate autoloader
+RUN composer dump-autoload
+
+# Default command
+CMD ["composer", "test"]