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/sentinel/desktop_content.html

@@ -0,0 +1,9 @@
+<div class="js-loading text-center text-xs text-stone-400">
+    loading… {{ page }} / {{ total_pages }}
+  </div>
+
+   <div class="js-neterr hidden inset-0 grid place-items-center p-4">
+    <div class="w-full max-w-[360px]">
+      {% include "sentinel/wireless_error.svg" %}
+    </div>
+  </div>