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>

README.md

@@ -2,7 +2,7 @@
 
 Stream music over ActivityPub - Build your own music streaming platform for Musicians and Labels.
 
-[![Version](https://img.shields.io/badge/version-0.5.1-blue.svg)](CHANGELOG.md)
+[![Version](https://img.shields.io/badge/version-0.6.0-blue.svg)](CHANGELOG.md)
 [![PHP](https://img.shields.io/badge/PHP-%3E%3D8.3-purple.svg)](https://php.net)
 [![WordPress](https://img.shields.io/badge/WordPress-%3E%3D6.4-blue.svg)](https://wordpress.org)
 [![License](https://img.shields.io/badge/license-GPL--2.0%2B-green.svg)](https://www.gnu.org/licenses/gpl-2.0.html)
@@ -146,7 +146,7 @@ wp-fedistream/
 │   ├── Plugin.php     # Main plugin singleton
 │   └── Installer.php  # Activation/deactivation
 ├── languages/         # Translation files
-├── templates/         # Twig templates
+├── templates/         # PHP templates
 ├── vendor/            # Composer dependencies
 ├── CHANGELOG.md       # Version history
 └── wp-fedistream.php  # Plugin entry point