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)

docker-compose.yml

@@ -0,0 +1,46 @@
+version: '3.8'
+
+services:
+  app:
+    build:
+      context: .
+      dockerfile: Dockerfile
+    volumes:
+      - .:/app
+      - composer-cache:/root/.composer/cache
+    environment:
+      - WP_TESTS_DIR=/tmp/wordpress-tests-lib
+    command: >
+      sh -c "
+        composer install --no-interaction &&
+        composer test
+      "
+
+  phpstan:
+    build:
+      context: .
+      dockerfile: Dockerfile
+    volumes:
+      - .:/app
+      - composer-cache:/root/.composer/cache
+    command: >
+      sh -c "
+        composer install --no-interaction &&
+        composer phpstan
+      "
+
+  lint:
+    build:
+      context: .
+      dockerfile: Dockerfile
+    volumes:
+      - .:/app
+      - composer-cache:/root/.composer/cache
+    command: >
+      sh -c "
+        composer install --no-interaction &&
+        composer lint
+      "
+
+volumes:
+  composer-cache: