refactor: migrate templates and static files to self-contained modules

Templates Migration:
- Migrate admin templates to modules (tenancy, billing, monitoring, marketplace, etc.)
- Migrate vendor templates to modules (tenancy, billing, orders, messaging, etc.)
- Migrate storefront templates to modules (catalog, customers, orders, cart, checkout, cms)
- Migrate public templates to modules (billing, marketplace, cms)
- Keep shared templates in app/templates/ (base.html, errors/, partials/, macros/)
- Migrate letzshop partials to marketplace module

Static Files Migration:
- Migrate admin JS to modules: tenancy (23 files), core (5 files), monitoring (1 file)
- Migrate vendor JS to modules: tenancy (4 files), core (2 files)
- Migrate shared JS: vendor-selector.js to core, media-picker.js to cms
- Migrate storefront JS: storefront-layout.js to core
- Keep framework JS in static/ (api-client, utils, money, icons, log-config, lib/)
- Update all template references to use module_static paths

Naming Consistency:
- Rename static/platform/ to static/public/
- Rename app/templates/platform/ to app/templates/public/
- Update all extends and static references

Documentation:
- Update module-system.md with shared templates documentation
- Update frontend-structure.md with new module JS organization

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/templates/admin/base.html

@@ -104,7 +104,7 @@
     <script src="{{ url_for('static', path='shared/js/icons.js') }}"></script>
 
     <!-- 3. THIRD: Alpine.js Base Data -->
-    <script src="{{ url_for('static', path='admin/js/init-alpine.js') }}"></script>
+    <script src="{{ url_for('core_static', path='admin/js/init-alpine.js') }}"></script>
 
     <!-- 4. FOURTH: Utils (standalone utilities) -->
     <script src="{{ url_for('static', path='shared/js/utils.js') }}"></script>
@@ -128,7 +128,7 @@
     </script>
 
     <!-- 7. SEVENTH: Vendor Selector (depends on Tom Select and API Client) -->
-    <script src="{{ url_for('static', path='shared/js/vendor-selector.js') }}"></script>
+    <script src="{{ url_for('core_static', path='shared/js/vendor-selector.js') }}"></script>
 
     <!-- 8a. Alpine.js Collapse Plugin (must load before Alpine) -->
     <script defer src="https://cdn.jsdelivr.net/npm/@alpinejs/collapse@3.13.3/dist/cdn.min.js"></script>