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>

includes/Frontend/template-wrapper.php

@@ -1,6 +1,6 @@
 <?php
 /**
- * Template wrapper for FediStream Twig templates.
+ * Template wrapper for FediStream PHP templates.
  *
  * @package WP_FediStream
  */