fix(email): support attachments, and stop emailTestMode='false' swallowing every email

[MaxAdams98]

Two independent problems in sendEmail, both found while building a monthly report that has to reach a government
contract manager with a signed PDF attached.

sendEmail cannot attach a file

There is no way to pass one, so an app that needs an attachment has to bypass sendEmail entirely and build the
Mailgun message itself, losing the template rendering, the recipient variables and the subject extraction.

nodemailer-mailgun-transport already reads mail.attachments and maps it onto Mailgun's attachment / inline
fields, so this is just a matter of passing the option through. The shape is nodemailer's, and an entry carrying a
cid is embedded inline rather than attached.

await sendEmail({
  config, template: 'report-submission', to: 'Thomas<thomas@example.gov>',
  attachments: [{ filename: 'monthly-2026-07.pdf', content: buffer, contentType: 'application/pdf' }],
})

emailTestMode=false reads as true, and the app silently sends nothing

config.emailTestMode is almost always populated from process.env, where every value is a string, and the
example app's server/config.js passes it straight through:

emailTestMode: process.env.emailTestMode,

const isTest = config.emailTestMode || test then treats the string 'false' as true. The app stays in test mode,
every sendEmail call resolves happily with the rendered HTML, and nothing is ever delivered. There is no error to
notice, which is what makes it worth fixing rather than documenting: an app that writes the flag out in full rather
than leaving it unset never sends an email and has no way to tell.

I hit this in production code that had been "sending" for weeks. The flag is now coerced, with the words that mean
off spelled out ('', false, 0, no, off); everything else keeps normal truthiness, so passing a real boolean
behaves exactly as before.

Also

config.emailTestMode was read one line before if (!config) throw, so calling sendEmail without a config threw
a TypeError instead of the intended message. The guard now comes first.

Verified

Patched into a real app's node_modules and exercised against it: the string 'false' now falls through to the
real send path rather than being swallowed, and attachments reach the transport. Sending end to end is blocked only
by that project's Mailgun key, which is read-only.

Independent of #6, which touches different lines of the same file.