♻️(mail) refactor mail to use npm instead of yarn

Following @manuhabitela's recommendation, we've decided to switch
from Yarn to npm for managing our Node.js dependencies. This update
aligns all remaining parts of the codebase that were still using
Yarn to now utilize npm.

Makefile

@@ -48,7 +48,7 @@ WAIT_DB             = @$(COMPOSE_RUN) dockerize -wait tcp://$(DB_HOST):$(DB_PORT
 
 # -- Backend
 MANAGE              = $(COMPOSE_RUN_APP) python manage.py
-MAIL_YARN           = $(COMPOSE_RUN) -w /app/src/mail node yarn # FIXME : use npm
+MAIL_NPM            = $(COMPOSE_RUN) -w /app/src/mail node npm
 
 # -- Frontend
 PATH_FRONT          = ./src/frontend
@@ -259,19 +259,19 @@ i18n-generate-and-upload: \
 # -- Mail generator
 
 mails-build: ## Convert mjml files to html and text
-	@$(MAIL_YARN) build
+	@$(MAIL_NPM) run build
 .PHONY: mails-build
 
 mails-build-html-to-plain-text: ## Convert html files to text
-	@$(MAIL_YARN) build-html-to-plain-text
+	@$(MAIL_NPM) run build-html-to-plain-text
 .PHONY: mails-build-html-to-plain-text
 
 mails-build-mjml-to-html:	## Convert mjml files to html and text
-	@$(MAIL_YARN) build-mjml-to-html
+	@$(MAIL_NPM) run build-mjml-to-html
 .PHONY: mails-build-mjml-to-html
 
 mails-install: ## install the mail generator
-	@$(MAIL_YARN) install
+	@$(MAIL_NPM) install
 .PHONY: mails-install