feat: Replace Twig with native PHP templates

- Remove twig/twig dependency from composer.json
- Convert all 25 Twig templates to native PHP templates
- New render() method in Plugin.php using PHP include with output buffering
- New render_partial() helper method for including partials
- Templates support theme overrides via fedistream/ directory
- Reduced plugin size by eliminating Twig and its dependencies

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

composer.json

@@ -22,8 +22,7 @@
     ],
     "require": {
         "php": ">=8.3",
-        "magdev/wc-licensed-product-client": "^0.2",
-        "twig/twig": "^3.0"
+        "magdev/wc-licensed-product-client": "^0.2"
     },
     "require-dev": {
         "phpunit/phpunit": "^10.0",