refactor: rename platform_domain → main_domain to avoid confusion with platform.domain

The setting `settings.platform_domain` (the global/main domain like "wizard.lu")
was easily confused with `platform.domain` (per-platform domain like "rewardflow.lu").
Renamed to `settings.main_domain` / `MAIN_DOMAIN` env var across the entire codebase.

Also updated docs to reflect the refactored store detection logic with
`is_platform_domain` / `is_subdomain_of_platform` guards.

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

app/modules/core/utils/page_context.py

@@ -205,7 +205,7 @@ def _build_base_context(
         "request": request,
         "platform": platform,
         "platform_name": settings.project_name,
-        "platform_domain": settings.platform_domain,
+        "main_domain": settings.main_domain,
     }
 
     # Add i18n globals