feat(email): let self-hosters brand transactional emails

[faizan47]

Fixes #444.

Most of Quackback white-labels fine already: workspace name, logo, colors, custom domain. The transactional emails were the gap. A handful of them hardcode "Quackback" in the subject and the heading, so an install running under its own brand still sends sign-in and password-reset mail that names the platform in the recipient's inbox, the one surface none of the in-app theming reaches.

The notification emails (status change, new comment, changelog) already read the workspace name and were fine. This only touches the auth and onboarding set.

What changed

A new EMAIL_BRAND_NAME env var, defaulting to "Quackback". When set, it replaces the hardcoded product name in:

• Subjects: sign-in link, sign-in refusal, password reset, invitation, welcome
• Headings and body: "Sign in to X", "Welcome to X!", "The X Team", "Reset your X password", "invited to join Acme on X"

It's passed as a brandName prop into the templates (default "Quackback"), so the components stay self-contained and existing installs render exactly as before.

I kept it separate from workspaceName on purpose. workspaceName is the board the mail is about; the brand name is the product it's sent from. Keeping them distinct means "join Acme on Quackback" still reads right instead of collapsing to "join Acme on Acme".

I left the "Powered by Quackback" footer alone, since that's the existing attribution link with its own toggle and seemed out of scope here.

On the config surface

I went with an env var because the email package is already env-configured (EMAIL_FROM, SMTP, SES), and it keeps the change contained with no call-site changes. If you'd rather source the name from settings so it can be set in the admin UI, I'm glad to switch it over. Just say so.

Testing

• New email-brand-name.test.ts covering the custom brand and the "Quackback" default for each affected template
• Full email package suite passes (184 tests), including the existing logo and signup tests
• prettier, oxlint, and the email package tsc --noEmit are clean

Documented the var in .env.example and .env.prod.example.

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[faizan47]

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