use node: prefix for requires

mostly because code is being autogenerated by all the AI stuff using
this prefix. it's also used in the stack trace.

src/mailer.js

@@ -20,7 +20,7 @@ exports = module.exports = {
     _mailQueue: [] // accumulate mails in test mode
 };
 
-const assert = require('assert'),
+const assert = require('node:assert'),
     BoxError = require('./boxerror.js'),
     branding = require('./branding.js'),
     constants = require('./constants.js'),
@@ -29,7 +29,7 @@ const assert = require('assert'),
     ejs = require('ejs'),
     mailServer = require('./mailserver.js'),
     nodemailer = require('nodemailer'),
-    path = require('path'),
+    path = require('node:path'),
     safe = require('safetydance'),
     translations = require('./translations.js');