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/master.blade.php

@@ -36,6 +36,7 @@
                 <a href="/likes">Likes</a>
                 <a href="/contacts">Contacts</a>
                 <a href="/projects">Projects</a>
+                <a href="/about">About</a>
                 <a href="{{ route('feed.notes.json') }}" class="rss-icon">@include('icons.json-feed', ['title' => 'JSON Feed'])</a>
             </nav>
             <div id="theme-selector" role="region" aria-label="Theme switcher">