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>

_data/recentComments.js

@@ -0,0 +1,24 @@
+/**
+ * Recent Comments Data
+ * Fetches the 5 most recent comments at build time for the sidebar widget.
+ */
+
+import EleventyFetch from "@11ty/eleventy-fetch";
+
+const INDIEKIT_URL = process.env.SITE_URL || "https://example.com";
+
+export default async function () {
+  try {
+    const url = `${INDIEKIT_URL}/comments/api/comments?limit=5`;
+    console.log(`[recentComments] Fetching: ${url}`);
+    const data = await EleventyFetch(url, {
+      duration: "15m",
+      type: "json",
+    });
+    console.log(`[recentComments] Got ${(data.children || []).length} comments`);
+    return data.children || [];
+  } catch (error) {
+    console.log(`[recentComments] Unavailable: ${error.message}`);
+    return [];
+  }
+}