Add modular app structure for L2 server

- Create app factory with routers and templates
- Auth, assets, activities, anchors, storage, users, renderers routers
- Federation router for WebFinger and nodeinfo
- Jinja2 templates for L2 pages
- Config and dependency injection

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/templates/404.html

@@ -0,0 +1,11 @@
+{% extends "base.html" %}
+
+{% block title %}Not Found - Art-DAG{% endblock %}
+
+{% block content %}
+<div class="text-center py-16">
+    <h2 class="text-6xl font-bold text-gray-600 mb-4">404</h2>
+    <p class="text-xl text-gray-400 mb-8">Page not found</p>
+    <a href="/" class="text-blue-400 hover:text-blue-300">Go to home page</a>
+</div>
+{% endblock %}