feat: add comment system components and recent comments widget

- Comment area on post pages (IndieAuth sign-in, submit, display)
- Alpine.js client-side component for auth flow and comment CRUD
- Recent comments sidebar widget with build-time data fetching
- Include comments.js in base layout, comments.njk before webmentions

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

_includes/components/widgets/recent-comments.njk

@@ -0,0 +1,25 @@
+{# Recent Comments Widget — sidebar #}
+{% if recentComments and recentComments.length %}
+<div class="sidebar-widget">
+  <h3 class="text-sm font-semibold text-surface-600 dark:text-surface-400 uppercase tracking-wide mb-3">Recent Comments</h3>
+  <ul class="space-y-3">
+    {% for comment in recentComments %}
+    <li class="text-sm">
+      <div class="flex items-start gap-2">
+        {% if comment.author and comment.author.photo %}
+        <img src="{{ comment.author.photo }}" alt="{{ comment.author.name }}"
+             class="w-6 h-6 rounded-full flex-shrink-0 mt-0.5" loading="lazy">
+        {% endif %}
+        <div>
+          <span class="font-medium">{{ comment.author.name or "Anonymous" }}</span>
+          <p class="text-surface-600 dark:text-surface-400 line-clamp-2">{{ comment.content.text | truncate(80) }}</p>
+          {% if comment["comment-target"] %}
+          <a href="{{ comment['comment-target'] }}" class="text-xs text-surface-500 hover:underline">View post</a>
+          {% endif %}
+        </div>
+      </div>
+    </li>
+    {% endfor %}
+  </ul>
+</div>
+{% endif %}