Add custom 404 error page

- Create custom 404 page matching blog layout and Eric's voice
- Includes helpful navigation links to home, tags, weeknotes, search
- References blog migrations since 2001 in a characteristically honest way
- Server-side nginx config added to blog.kestrelsnest.social.d/custom_errors.conf

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

layouts/404.html

@@ -0,0 +1,24 @@
+{{ define "main" }}
+  <article class="post">
+    <header class="post-header">
+      <h1 class="post-title">404: Page Not Found</h1>
+    </header>
+
+    <div class="post-content">
+      <p>Well, this is awkward.</p>
+
+      <p>The page you're looking for isn't here. Maybe it never was, or maybe it got lost in one of the many migrations this blog has been through since 2001.</p>
+
+      <p>Here are some places you might want to try:</p>
+
+      <ul>
+        <li><a href="/">Home</a> — Recent posts</li>
+        <li><a href="/tags/">Tags</a> — Browse by topic</li>
+        <li><a href="/weeknotes/">Weeknotes</a> — Weekly updates</li>
+        <li><a href="/search/">Search</a> — Find what you're looking for</li>
+      </ul>
+
+      <p>Or just <a href="mailto:eric@ericwagoner.com?subject=Broken link on blog">let me know</a> if you think something should be here.</p>
+    </div>
+  </article>
+{{ end }}