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>

templates/archive/playlist.php

@@ -0,0 +1,49 @@
+<?php
+/**
+ * Playlist archive template.
+ *
+ * @package WP_FediStream
+ *
+ * @var array  $posts              Array of playlist data.
+ * @var string $archive_title      Archive title.
+ * @var string $archive_description Archive description.
+ * @var array  $pagination         Pagination data.
+ */
+
+// Prevent direct file access.
+if ( ! defined( 'ABSPATH' ) ) {
+    exit;
+}
+
+$plugin = \WP_FediStream\Plugin::get_instance();
+?>
+<div class="fedistream-archive fedistream-archive--playlists">
+    <header class="fedistream-archive__header">
+        <h1 class="fedistream-archive__title"><?php esc_html_e( 'Playlists', 'wp-fedistream' ); ?></h1>
+        <?php if ( ! empty( $archive_description ) ) : ?>
+            <div class="fedistream-archive__description"><?php echo wp_kses_post( $archive_description ); ?></div>
+        <?php endif; ?>
+    </header>
+
+    <?php if ( ! empty( $posts ) && is_array( $posts ) ) : ?>
+        <div class="fedistream-grid fedistream-grid--playlists">
+            <?php foreach ( $posts as $post ) : ?>
+                <?php echo $plugin->render_partial( 'partials/card-playlist', array( 'post' => $post ) ); // phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped ?>
+            <?php endforeach; ?>
+        </div>
+
+        <?php if ( ! empty( $pagination['links'] ) ) : ?>
+            <nav class="fedistream-pagination">
+                <?php
+                foreach ( $pagination['links'] as $link ) {
+                    echo $link; // phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped
+                }
+                ?>
+            </nav>
+        <?php endif; ?>
+    <?php else : ?>
+        <div class="fedistream-empty">
+            <p><?php esc_html_e( 'No playlists found.', 'wp-fedistream' ); ?></p>
+        </div>
+    <?php endif; ?>
+</div>