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)

wp-recache.php

@@ -0,0 +1,62 @@
+<?php
+/**
+ * Plugin Name: WP Recache
+ * Plugin URI: https://gitea.cyanet.fr/kevin.bataille/wp-recache
+ * Description: Open source WordPress performance plugin - page caching, file optimization, image optimization
+ * Version: 1.0.0
+ * Requires at least: 5.8
+ * Requires PHP: 7.4
+ * Author: Kevin Bataille
+ * Author URI: https://gitea.cyanet.fr/kevin.bataille
+ * License: GPLv2 or later
+ * License URI: https://www.gnu.org/licenses/gpl-2.0.html
+ * Text Domain: wp-recache
+ * Domain Path: /languages
+ */
+
+defined('ABSPATH') || exit;
+
+define('WP_RECACHE_VERSION', '1.0.0');
+define('WP_RECACHE_PATH', plugin_dir_path(__FILE__));
+define('WP_RECACHE_URL', plugin_dir_url(__FILE__));
+define('WP_RECACHE_INC_PATH', WP_RECACHE_PATH . 'includes/');
+define('WP_RECACHE_CACHE_PATH', WP_CONTENT_DIR . '/cache/wp-recache/');
+
+require_once WP_RECACHE_INC_PATH . 'Common/Helpers.php';
+
+/**
+ * Main plugin initialization.
+ */
+function wp_recache_init() {
+    if (is_admin()) {
+        require_once WP_RECACHE_INC_PATH . 'Admin/Admin.php';
+        $admin = new WP_Recache_Admin();
+        $admin->init();
+    }
+
+    if (WP_RECACHE_CACHE_ENABLED) {
+        require_once WP_RECACHE_INC_PATH . 'Cache/WPCache.php';
+        $cache = new WP_Recache_WPCache();
+        $cache->init();
+    }
+}
+
+/**
+ * Plugin activation.
+ */
+function wp_recache_activate() {
+    require_once WP_RECACHE_INC_PATH . 'Activator.php';
+    WP_Recache_Activator::activate();
+}
+register_activation_hook(__FILE__, 'wp_recache_activate');
+
+/**
+ * Plugin deactivation.
+ */
+function wp_recache_deactivate() {
+    require_once WP_RECACHE_INC_PATH . 'Deactivator.php';
+    WP_Recache_Deactivator::deactivate();
+}
+register_deactivation_hook(__FILE__, 'wp_recache_deactivate');
+
+add_action('plugins_loaded', 'wp_recache_init');