Fix multiple open issues: logging, peer deps, addTransporter, CSS inline bundling, MJML docs

[Copilot]

Addresses several open issues across logging, dependency resolution, code consistency, webpack compatibility, and documentation.

Changes

• verifyTransporter log level (Path: **lib/mailer.service.ts** #1239) — Changed .debug() → .log() so "Transporter is ready" is visible at default log levels instead of silently swallowed

• Optional peer dependencies (not working with nestjs 11 #1244) — Added peerDependenciesMeta marking template engines (ejs, handlebars, mjml, pug, liquidjs), their type packages, and preview-email as optional. Fixes npm install failures with NestJS 11+ when not all engines are installed

• addTransporter consistency (Hook doesn't run when using custom configure transport #1234) — Refactored to reuse createTransporter() instead of duplicating transport creation inline. Ensures verification and template adapter init follow the same code path as setupTransporters()

• Lazy-load @css-inline/css-inline (Error triggered using mailer and any template adapter combined with a custom webpack.config.js, reporting that dist/css_inline_bg.wasm not found. #1095) — Switched from static top-level import to require() at call site in handlebars, ejs, and pug adapters. The .wasm dependency is now only resolved when CSS inlining actually executes, fixing webpack bundling errors

• MJML documentation (Documentation is a mess #1246) — Clarified why inlineCssEnabled: false is required (MJML handles its own CSS inlining) and added note on Handlebars partials with MJML

Example: lazy CSS inline loading

// Before — fails at import time if .wasm not bundled
import { inline } from '@css-inline/css-inline';

// After — only fails if inlineCssEnabled is true and module is missing
if (this.config.inlineCssEnabled) {
  const { inline } = require('@css-inline/css-inline');
  mail.data.html = inline(rendered, this.config.inlineCssOptions);
}

Not addressed (out of scope)

• html-minifier - kangax html-minifier REDoS vulnerability #1241 (html-minifier vuln) — transitive dep of mjml 4.x; optionalDependencies already targets mjml 5.x
• Vulnerability cross-spawn #1236 (cross-spawn vuln) — dev-only transitive dep, not shipped to consumers

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