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)

blog/templates/fragments/nav_tree.html

@@ -0,0 +1,32 @@
+{# Nav-tree fragment — rendered by blog, consumed by all apps.
+   Uses frag_app_name / frag_first_seg instead of request.path / app_name
+   so the consuming app's context is reflected correctly.
+   No hx-boost — cross-app nav links are full page navigations. #}
+{% set _app_slugs = {
+  'cart': cart_url('/'),
+  'market': market_url('/'),
+  'events': events_url('/'),
+  'federation': federation_url('/'),
+  'account': account_url('/'),
+} %}
+<div class="flex flex-col sm:flex-row sm:items-center gap-2 border-r border-stone-200 mr-2 sm:max-w-2xl"
+     id="menu-items-nav-wrapper">
+  {% from 'macros/scrolling_menu.html' import scrolling_menu with context %}
+  {% call(item) scrolling_menu('menu-items-container', menu_items) %}
+    {% set _href = _app_slugs.get(item.slug, blog_url('/' + item.slug + '/')) %}
+    <a
+      href="{{ _href }}"
+      aria-selected="{{ 'true' if (item.slug == frag_first_seg or item.slug == frag_app_name) else 'false' }}"
+      class="{{styles.nav_button_less_pad}}"
+    >
+      {% if item.feature_image %}
+        <img src="{{ item.feature_image }}"
+             alt="{{ item.label }}"
+             class="w-8 h-8 rounded-full object-cover flex-shrink-0" />
+      {% else %}
+        <div class="w-8 h-8 rounded-full bg-stone-200 flex-shrink-0"></div>
+      {% endif %}
+      <span>{{ item.label }}</span>
+    </a>
+  {% endcall %}
+</div>