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/auth/already_logged_in.html

@@ -0,0 +1,12 @@
+{% extends "base.html" %}
+
+{% block title %}Already Logged In - Art-DAG{% endblock %}
+
+{% block content %}
+<div class="max-w-md mx-auto text-center">
+    <div class="bg-green-900/50 border border-green-700 text-green-300 px-4 py-3 rounded-lg mb-4">
+        You are already logged in as <strong>{{ user.username }}</strong>
+    </div>
+    <p><a href="/" class="text-blue-400 hover:text-blue-300">Go to home page</a></p>
+</div>
+{% endblock %}