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/billing/services/signup_service.py

@@ -617,7 +617,7 @@ class SignupService:
 
             # Build login URL
             login_url = (
-                f"https://{settings.platform_domain}"
+                f"https://{settings.main_domain}"
                 f"/store/{store.store_code}/dashboard"
             )