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)

phpstan.neon

@@ -0,0 +1,11 @@
+includes:
+    - vendor/phpstan/phpstan-de-phpstan-wordpress
+
+parameters:
+    level: 5
+    paths:
+        - includes/
+        - wp-recache.php
+    tmpDir: .phpstan-cache
+    ignoreErrors:
+        - '#Function wp_recache_.* not found\.#'