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/widgets/popular-tracks.php

@@ -0,0 +1,45 @@
+<?php
+/**
+ * Popular Tracks Widget Template.
+ *
+ * @package WP_FediStream
+ *
+ * @var array $posts Array of track data.
+ */
+
+// Prevent direct file access.
+if ( ! defined( 'ABSPATH' ) ) {
+    exit;
+}
+?>
+<?php if ( ! empty( $posts ) && is_array( $posts ) ) : ?>
+    <ol class="fedistream-widget__list fedistream-widget__list--tracks">
+        <?php foreach ( $posts as $post ) : ?>
+            <li class="fedistream-widget__item" data-track-id="<?php echo esc_attr( $post['id'] ?? '' ); ?>">
+                <a href="<?php echo esc_url( $post['permalink'] ?? '#' ); ?>" class="fedistream-widget__link">
+                    <?php if ( ! empty( $post['thumbnail'] ) ) : ?>
+                        <img src="<?php echo esc_url( $post['thumbnail'] ); ?>" alt="<?php echo esc_attr( $post['title'] ?? '' ); ?>" class="fedistream-widget__image">
+                    <?php else : ?>
+                        <span class="fedistream-widget__placeholder">
+                            <svg viewBox="0 0 24 24" fill="currentColor"><path d="M12 3v10.55c-.59-.34-1.27-.55-2-.55-2.21 0-4 1.79-4 4s1.79 4 4 4 4-1.79 4-4V7h4V3h-6z"/></svg>
+                        </span>
+                    <?php endif; ?>
+                    <span class="fedistream-widget__info">
+                        <span class="fedistream-widget__title"><?php echo esc_html( $post['title'] ?? '' ); ?></span>
+                        <?php if ( ! empty( $post['artist'] ) ) : ?>
+                            <span class="fedistream-widget__artist"><?php echo esc_html( $post['artist'] ); ?></span>
+                        <?php endif; ?>
+                    </span>
+                    <?php if ( ! empty( $post['play_count'] ) ) : ?>
+                        <span class="fedistream-widget__plays"><?php echo esc_html( number_format( $post['play_count'] ) ); ?></span>
+                    <?php endif; ?>
+                </a>
+                <button type="button" class="fedistream-widget__play" data-track-id="<?php echo esc_attr( $post['id'] ?? '' ); ?>" aria-label="<?php esc_attr_e( 'Play', 'wp-fedistream' ); ?>">
+                    <svg viewBox="0 0 24 24" fill="currentColor"><path d="M8 5v14l11-7z"/></svg>
+                </button>
+            </li>
+        <?php endforeach; ?>
+    </ol>
+<?php else : ?>
+    <p class="fedistream-widget__empty"><?php esc_html_e( 'No tracks yet.', 'wp-fedistream' ); ?></p>
+<?php endif; ?>