Add email sending to a Lovable app

Add transactional email to this app using emails.sh, so a new signup gets a verification email.

Read https://emails.sh/docs.md first so you use the real request shape instead of guessing it.

Requirements:
- Do the send in backend code (an edge function), never in a React component. The API key must not appear in any file that ships to the browser, and must not use the VITE_ prefix.
- Read the key from a secret named EMAILSSH_API_KEY. I will add it under Secrets in the project's Cloud panel. If it is not set, fail with a clear error message rather than sending without it.
- Call POST https://emails.sh/v1/emails with an Authorization: Bearer header, from onboarding@emails.sh for now.
- Add a form that triggers it, and tell me exactly which secret name you expect.

// Runs on the server. The key is read from the project's secrets, so it is
// never part of anything the browser downloads.
Deno.serve(async (request) => {
	const apiKey = Deno.env.get('EMAILSSH_API_KEY');
	if (!apiKey) {
		// Fail loudly. A silent no-op here looks like a deliverability problem
		// three days later.
		return new Response('EMAILSSH_API_KEY is not set', { status: 500 });
	}

	const { email, token } = await request.json();
	const link = `https://acme.com/verify?token=${token}`;

	const response = await fetch('https://emails.sh/v1/emails', {
		method: 'POST',
		headers: {
			Authorization: `Bearer ${apiKey}`,
			'Content-Type': 'application/json'
		},
		body: JSON.stringify({
			from: 'Acme <onboarding@emails.sh>',
			to: [email],
			subject: 'Confirm your email',
			html: `<p><a href="${link}">Confirm your email</a></p>`,
			text: `Confirm your email: ${link}`
		})
	});

	// { "id": "...", "status": "queued" } on success. On a 4xx the body says
	// what to fix in prose, so pass it through rather than swallowing it.
	const result = await response.json();
	return new Response(JSON.stringify(result), {
		status: response.status,
		headers: { 'Content-Type': 'application/json' }
	});
});

To:      signup@example.com
Subject: Confirm your email

Confirm your email: https://acme.com/verify?token=af31c0