Use coop_url for auth links in shared templates

url_for('auth.account') and url_for('auth.newsletters') only work in
the blog app. Other apps (cart) that render these shared templates
don't have an auth blueprint. Use coop_url() so links always resolve
to the blog subdomain regardless of which app renders the template.

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

browser/templates/_types/auth/_nav.html

@@ -1,5 +1,5 @@
 {% import 'macros/links.html' as links %}
-{% call links.link(url_for('auth.newsletters'), hx_select_search, select_colours, True, aclass=styles.nav_button) %}
+{% call links.link(coop_url('/auth/newsletters/'), hx_select_search, select_colours, True, aclass=styles.nav_button) %}
   newsletters
 {% endcall %}
 <div class="relative nav-group">

browser/templates/_types/auth/header/_header.html

@@ -1,7 +1,7 @@
 {% import 'macros/links.html' as links %}
 {% macro header_row(oob=False) %}
   {% call links.menu_row(id='auth-row', oob=oob) %}
-    {% call links.link(url_for('auth.account'), hx_select_search  ) %}
+    {% call links.link(coop_url('/auth/account/'), hx_select_search  ) %}
       <i class="fa-solid fa-user"></i>
       <div>account</div>
     {% endcall %}