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

app/Http/Controllers/AboutPageController.php

@@ -0,0 +1,16 @@
+<?php
+
+namespace App\Http\Controllers;
+
+use App\Models\About;
+use Illuminate\View\View;
+
+class AboutPageController extends Controller
+{
+    public function show(): View
+    {
+        return view('about', [
+            'about' => About::first()?->content,
+        ]);
+    }
+}