Add admin-editable About page

Mirrors the existing Bio singleton pattern: a new `about` table/model,
admin CRUD at /admin/about, and a public page at /about linked from
both the header nav and the admin homepage. Content is wrapped in
.e-content so it lays out correctly within the site's CSS grid.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013bzYwaDD8p3XNDMYKvXKrs

resources/views/admin/welcome.blade.php

@@ -57,6 +57,11 @@
         Edit your <a href="/admin/bio">bio</a>.
     </p>
 
+    <h2>About</h2>
+    <p>
+        Edit your <a href="/admin/about">about page</a>.
+    </p>
+
     <h2>Passkeys</h2>
     <p>
         Manager <a href="/admin/passkeys">your passkeys</a>.