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

tests/Feature/AboutPageTest.php

@@ -0,0 +1,26 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\Feature;
+
+use App\Models\About;
+use Illuminate\Foundation\Testing\RefreshDatabase;
+use PHPUnit\Framework\Attributes\Test;
+use Tests\TestCase;
+
+class AboutPageTest extends TestCase
+{
+    use RefreshDatabase;
+
+    #[Test]
+    public function about_page_shows_content(): void
+    {
+        About::factory()->create([
+            'content' => 'This is the about page content.',
+        ]);
+
+        $this->get('/about')
+            ->assertSee('This is the about page content.');
+    }
+}