feat: initial shared library extraction

Contains shared infrastructure for all coop services:
- shared/ (factory, urls, user_loader, context, internal_api, jinja_setup)
- models/ (User, Order, Calendar, Ticket, Product, Ghost CMS)
- db/ (SQLAlchemy async session, base)
- suma_browser/app/ (csrf, middleware, errors, authz, redis_cacher, payments, filters, utils)
- suma_browser/templates/ (shared base layouts, macros, error pages)
- static/ (CSS, JS, fonts, images)
- alembic/ (database migrations)
- config/ (app-config.yaml)
- editor/ (Lexical editor Node.js build)
- requirements.txt

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

suma_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 %}