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>

env-example

@@ -2,8 +2,6 @@ NODE_ENV=development
 APP_PORT=3000
 APP_NAME="NestJS API"
 API_PREFIX=api
-APP_FALLBACK_LANGUAGE=en
-APP_HEADER_LANGUAGE=x-custom-lang
 FRONTEND_DOMAIN=http://localhost:3000
 BACKEND_DOMAIN=http://localhost:3000