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/src/mail/mail-config.service.ts

@@ -1,4 +1,6 @@
 import * as path from 'path';
+import * as fs from 'fs';
+import * as handlebars from 'handlebars';
 import { Injectable } from '@nestjs/common';
 import { ConfigService } from '@nestjs/config';
 import { MailerOptions, MailerOptionsFactory } from '@nestjs-modules/mailer';
@@ -9,6 +11,21 @@ export class MailConfigService implements MailerOptionsFactory {
   constructor(private configService: ConfigService) {}
 
   createMailerOptions(): MailerOptions {
+    const templatesDir = path.join(
+      this.configService.get('app.workingDirectory'),
+      'src',
+      'mail',
+      'mail-templates',
+    );
+
+    handlebars.registerPartial(
+      'layout',
+      fs.readFileSync(
+        path.join(templatesDir, 'partials', 'layout.hbs'),
+        'utf-8',
+      ),
+    );
+
     return {
       transport: {
         host: this.configService.get('mail.host'),
@@ -27,24 +44,10 @@ export class MailConfigService implements MailerOptionsFactory {
         )}" <${this.configService.get('mail.defaultEmail')}>`,
       },
       template: {
-        dir: path.join(
-          this.configService.get('app.workingDirectory'),
-          'src',
-          'mail',
-          'mail-templates',
-        ),
+        dir: templatesDir,
         adapter: new HandlebarsAdapter(),
         options: {
           strict: true,
-          partials: {
-            dir: path.join(
-              this.configService.get('app.workingDirectory'),
-              'src',
-              'mail',
-              'mail-templates',
-              'partials',
-            ),
-          },
         },
       },
     } as MailerOptions;