fix: SMTP confirmation

Changes:

1. add NEXT_PUBLIC_API_URL environment for front ( STRAPI_URL use the intranet http://strapi:1337, NEXT_PUBLIC_API_URL use https://api.dev.voca.city).
2. add notifications on smtp credentials saving:

• when email is sent: a warning to say to check the mails
• when email can't be sent: an error
• when the confirmation is done: a success message

3. Froze the SMTP form when the email is sent: If you edit again and again smtp credentials, you never know if what you are confirming is actually working. After checking with Lucien, we froze the form for 2min if the email have been sent. To do so, we check the last warning creation date to know for how long we are waiting.
4. SMTP notifications need to be relevant to the data we see. Before we show the last notification. It was not relevant, because if you successfully update you credentials, you should see the notification again. => We display now only notifications more recent than the last credential update.