Replace L2 JWT auth with OAuth SSO via account.rose-ash.com

- config.py: OAuth settings replace l2_server/l2_domain
- auth.py: full rewrite — login/callback/logout with itsdangerous
  signed state cookies and httpx token exchange
- dependencies.py: remove l2_server assignment, fix redirect path
- home.py: simplify /login to redirect to /auth/login
- base.html: cross-app nav (Blog, Market, Account) + Rose Ash branding
- requirements.txt: add itsdangerous

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

app/templates/base.html

@@ -1,6 +1,10 @@
 {% extends "_base.html" %}
 
-{% block brand %}Art-DAG L1{% endblock %}
+{% block brand %}
+<a href="https://blog.rose-ash.com/" class="text-white hover:text-gray-200 no-underline">Rose Ash</a>
+<span class="text-gray-500 mx-1">|</span>
+Art-DAG
+{% endblock %}
 
 {% block nav_items %}
 <nav class="flex items-center space-x-6">
@@ -10,6 +14,10 @@
     <a href="/media" class="text-gray-300 hover:text-white {% if active_tab == 'media' %}text-white font-medium{% endif %}">Media{% if nav_counts and nav_counts.media %} ({{ nav_counts.media }}){% endif %}</a>
     <a href="/storage" class="text-gray-300 hover:text-white {% if active_tab == 'storage' %}text-white font-medium{% endif %}">Storage{% if nav_counts and nav_counts.storage %} ({{ nav_counts.storage }}){% endif %}</a>
     <a href="/download/client" class="text-gray-300 hover:text-white" title="Download CLI client">Client</a>
+    <span class="text-gray-600">|</span>
+    <a href="https://blog.rose-ash.com/" class="text-gray-400 hover:text-white text-sm">Blog</a>
+    <a href="https://market.rose-ash.com/" class="text-gray-400 hover:text-white text-sm">Market</a>
+    <a href="https://account.rose-ash.com/" class="text-gray-400 hover:text-white text-sm">Account</a>
 </nav>
 {% endblock %}
 
@@ -20,6 +28,6 @@
     <a href="/auth/logout" class="text-gray-300 hover:text-white">Logout</a>
 </div>
 {% else %}
-<a href="/login" class="text-gray-300 hover:text-white">Login</a>
+<a href="/auth/login" class="text-gray-300 hover:text-white">Login</a>
 {% endif %}
 {% endblock %}