Monorepo: consolidate 7 repos into one

Combines shared, blog, market, cart, events, federation, and account
into a single repository. Eliminates submodule sync, sibling model
copying at build time, and per-app CI orchestration.

Changes:
- Remove per-app .git, .gitmodules, .gitea, submodule shared/ dirs
- Remove stale sibling model copies from each app
- Update all 6 Dockerfiles for monorepo build context (root = .)
- Add build directives to docker-compose.yml
- Add single .gitea/workflows/ci.yml with change detection
- Add .dockerignore for monorepo build context
- Create __init__.py for federation and account (cross-app imports)

shared/browser/templates/macros/admin_nav.html

@@ -0,0 +1,21 @@
+{#
+  Shared admin navigation macro
+  Use this instead of duplicate _nav.html files
+#}
+
+{% macro admin_nav_item(href, icon='cog', label='', select_colours='', aclass=styles.nav_button) %}
+  {% import 'macros/links.html' as links %}
+  {% call links.link(href, hx_select_search, select_colours, True, aclass=aclass) %}
+    <i class="fa fa-{{ icon }}" aria-hidden="true"></i>
+    {{ label }}
+  {% endcall %}
+{% endmacro %}
+
+{% macro placeholder_nav() %}
+{# Placeholder for admin sections without specific nav items #}
+<div class="relative nav-group">
+  <span class="block px-3 py-2 text-stone-400 text-sm italic">
+    Admin options
+  </span>
+</div>
+{% endmacro %}