Add About OOTT page with version, links, and legal notices

Implements the /about route with app description, version info,
clickable links to the source repo and AGPL-3.0 license, and
third-party copyright notices. Adds url_launcher for cross-platform
link handling.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/lib/navigation.dart

@@ -1,4 +1,5 @@
 import 'package:flutter/material.dart';
+import 'package:frontend/about/about.dart';
 import 'package:frontend/settings/settings.dart';
 import 'package:go_router/go_router.dart';
 import 'package:google_fonts/google_fonts.dart';
@@ -47,7 +48,7 @@ final GoRouter router = GoRouter(
         GoRoute(
           path: '/about',
           name: 'about',
-          builder: (context, state) => const Placeholder(),
+          builder: (context, state) => const About(),
         ),
       ],
     ),