fix: improve verification email deliverability and Outlook spam handling

[aminamos]

Summary:

Resolves #121.
Integrates transactional email service improvements (Brevo client), documentation in docs/email-delivery-brevo.md, and helper notices alerting Outlook users to check Junk folders.

Expected behavior:

Clear guidance and improved headers prevent verification emails from being silently dropped by Outlook filters.

Testing tips:

1. Checkout fix/121-outlook-verification-email.
2. Run yarn test.
3. Trigger email verification flow.

Please make sure these boxes are checked before submitting your pull request - thanks!

• Run the unit tests with yarn test to make sure you didn't break anything
• Add or update any needed documentation to the repo
• Format the title like "feat: [new feature short description]". Title must follow the Conventional Commit Specification(https://www.conventionalcommits.org/en/v1.0.0/).
• Linked all relevant issues
• Include screenshot(s) showing how this pull request works and fixes the issue(s)

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