fix: replace stale blog.post.* endpoint refs with coop_url()

Replace url_for('blog.post.post_detail', ...) and
url_for('blog.post.admin.entries', ...) with coop_url() for
cross-service links back to the blog app.

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

templates/_types/entry/_nav.html

@@ -6,7 +6,7 @@
   {% from 'macros/scrolling_menu.html' import scrolling_menu with context %}
   {% call(entry_post) scrolling_menu('entry-posts-container', entry_posts) %}
     <a
-      href="{{ url_for('blog.post.post_detail', slug=entry_post.slug) }}"
+      href="{{ coop_url('/' + entry_post.slug + '/') }}"
       class="flex items-center gap-2 px-3 py-2 hover:bg-stone-100 rounded transition text-sm border sm:whitespace-nowrap sm:flex-shrink-0">
       {% if entry_post.feature_image %}
         <img src="{{ entry_post.feature_image }}"

templates/_types/entry/admin/_nav_posts_oob.html

@@ -9,7 +9,7 @@
     {% from 'macros/scrolling_menu.html' import scrolling_menu with context %}
     {% call(entry_post) scrolling_menu('entry-posts-container', entry_posts) %}
       <a
-        href="{{ url_for('blog.post.post_detail', slug=entry_post.slug) }}"
+        href="{{ coop_url('/' + entry_post.slug + '/') }}"
         class="{{styles.nav_button}}"
       >
         {% if entry_post.feature_image %}

templates/_types/post_entries/header/_header.html

@@ -1,7 +1,7 @@
 {% import 'macros/links.html' as links %}
 {% macro header_row(oob=False) %}
   {% call links.menu_row(id='post_entries-row', oob=oob) %}
-    {% call links.link(url_for('blog.post.admin.entries', slug=post.slug), hx_select_search) %}
+    {% call links.link(coop_url('/' + post.slug + '/admin/entries/'), hx_select_search) %}
       <i class="fa fa-clock" aria-hidden="true"></i>
       <div>
         entries