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>

services/widget_registry.py

@@ -48,8 +48,8 @@ class _WidgetRegistry:
         slug = w.slug
 
         def _href(s=slug):
-            from shared.infrastructure.urls import coop_url
-            return coop_url(f"/auth/{s}/")
+            from shared.infrastructure.urls import blog_url
+            return blog_url(f"/auth/{s}/")
 
         self._account_nav.append(AccountNavLink(
             label=w.label,