docs: prepare deferred SMTP delivery setup (#152, #171)

[Muawiya-contact]

This draft prepares SMTP delivery configuration and real-inbox acceptance for #152 and #171. It retains the optional Resend/domain setup, the owner's existing Gmail choice, the app's redirect contract, provider limits, and troubleshooting guidance. The owner has a $0 budget and is not purchasing a domain now.

The three branded HTML templates and their provider-independent installation guide are delivered separately in ready-for-review PR #188. A follow-up commit removes the duplicated HTML from this draft and updates its references; the original commit history is preserved. #188 can merge independently of this deferred setup.

Keep this PR in draft. SMTP authentication, provider capacity, real inbox delivery, and actual email-client rendering remain unverified. No domain purchase, SMTP configuration, dashboard template installation, email sending, or production release was performed by this PR. Installing custom HTML does not remove Supabase's default sender restrictions; other recipients still require configured custom SMTP.

Validation for the separation: reviewed the documentation diff, verified local Markdown paths, and passed git diff --check. No backend/mobile or live-email tests were run for this documentation/removal change. The unchanged template source and its 60 local browser scenarios are recorded in #188.

Before marking this delivery draft ready:

• Configure and authenticate the selected sender in the correct Supabase project; record its limits and accepted rollout scope.
• Install the templates from feat: add branded Supabase account email templates #188 and verify existing redirect URLs.
• Verify signup and recovery through successful app sign-in, including a non-team recipient.
• Confirm the enabled password-changed notification arrives for a test account.
• Check a second inbox provider, fallback/expired links, authentication headers, and Supabase Auth errors; record redacted evidence.

Closing references apply only after the outstanding delivery work is accepted and this PR merges. The template PR does not close these delivery issues.

Closes #152
Closes #171