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/Models/About.php

@@ -0,0 +1,13 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Model;
+
+class About extends Model
+{
+    use HasFactory;
+
+    protected $table = 'about';
+}