fix: register mail layout partial on handlebars singleton for real mailer wiring

The HandlebarsAdapter reads partials config from a top-level sibling of
`template` (mailerOptions.options.partials), not from
template.options.partials where it was nested. Because the mail templates
use partial blocks ({{#> layout}}...{{/layout}}), the unregistered partial
rendered silently as an unstyled fragment instead of throwing, so this went
unnoticed. A config-only fix also breaks on Windows because the adapter's
glob-based directory loader mishandles backslash path separators.

Fix registers the shared `layout` partial directly on the handlebars module
singleton in MailConfigService, bypassing the broken glob loader entirely.

Also:
- add mail-config.service.spec.ts, an integration test that drives the real
  MailerOptions + HandlebarsAdapter wiring (would have caught this bug,
  unlike the existing template-only spec which registers the partial itself)
- remove stale nestjs-i18n references from .env.example, env-example, and
  the backend README (i18n was already removed from the code)
- add missing trailing newlines to activation.hbs and reset-password.hbs

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

myteamwallet_backend/README.md

@@ -26,7 +26,6 @@ Seeden: npm run seed:run
 - [x] Sign in and sign up via email.
 - [x] Social sign in (Apple, Facebook, Google, Twitter).
 - [x] Admin and User roles.
-- [x] I18N ([nestjs-i18n](https://www.npmjs.com/package/nestjs-i18n)).
 - [x] File uploads. Support local and Amazon S3 drivers.
 - [x] Swagger.
 - [x] E2E and units tests.