refactor(cms): migrate store theme UI from tenancy to CMS module

Move store theme admin pages, templates, and JS from tenancy module
to CMS module where the data layer (model, service, API, schemas)
already lives. Eliminates split ownership.

Moved:
- Route handlers: GET /store-themes, GET /stores/{code}/theme
- Templates: store-theme.html, store-themes.html
- JS: store-theme.js, store-themes.js
- Updated static references: tenancy_static → cms_static

Deleted old tenancy files (no remaining references).
Menu item in CMS definition already pointed to correct route.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/proposals/cms-redesign-alignment.md

@@ -242,7 +242,7 @@ This proposal covers the CMS foundation. The broader [storefront builder vision]
 | Section partials | `app/modules/cms/templates/cms/platform/sections/_*.html` |
 | CMS definition (admin menu) | `app/modules/cms/definition.py` |
 | Tenancy definition (admin menu) | `app/modules/tenancy/definition.py` |
-| Store theme page | `app/modules/tenancy/templates/tenancy/admin/store-theme.html` |
+| Store theme page | `app/modules/cms/templates/cms/admin/store-theme.html` |
 | Store themes list | `app/modules/cms/templates/cms/admin/store-themes.html` |
 | Storefront landing templates | `app/modules/cms/templates/cms/storefront/landing-*.html` |
 | Seed data | `scripts/seed/create_default_content_pages.py` |