Rename coop to blog throughout codebase

- coop_url() → blog_url(), AUTH_APP default → "blog"
- Session cookie: coop_session → blog_session
- Config keys: coop_root/coop_title → market_root/market_title
- All Jinja templates: coop_url → blog_url, coop_title → market_title
- Template blocks: coop-child-header → blog-child-header

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

browser/templates/macros/title.html

@@ -1,6 +1,6 @@
 {% macro title(_class='') %}
   <a
-    href="{{ coop_url('/') }}"
+    href="{{ blog_url('/') }}"
     class="{{_class}}"
   >
   <h1>