# Account Linking

## Objective

Let one account sign in several ways without spawning duplicates.

Attach and detach sign-in methods — password, OAuth providers, SSO — on a single existing account.

## Before You Begin

This feature is being added to an application that already exists and already
works. Do not scaffold a new project, and do not assume a blank slate.

Inspect the codebase first and establish:

- The existing application structure and where code of this kind already lives.
- The framework and version in use.
- The existing design system — colours, spacing, typography, and component conventions.
- Existing UI components you can reuse instead of writing new ones.
- The existing database structure, if this feature needs to persist anything.
- The existing authentication and authorization system, if this feature is user-scoped.
- Dependencies already installed, so you don't add a library that duplicates one.
- The existing test setup and conventions.

Only start writing code once you understand the above. If the application
already implements part of this feature, extend it rather than replacing it.

## Implementation Instructions

1. Model sign-in methods as separate identity records attached to one account, rather than columns on the user.
2. Require a fresh re-authentication before any link or unlink. A stale session is enough to read the app; it is not enough to change how the account is entered.
3. Refuse to link an identity that is already attached to another account. Explain the conflict and point the user at recovery — do not silently steal the identity or silently create a second account.
4. Before unlinking, prove at least one usable method remains. Unlinking the last one locks the user out of their own data.
5. Do NOT treat a matching email address as proof of ownership. Providers vary in whether they verify email, and an unverified match is a takeover route.
6. Combining two accounts that both already exist is owned by Account Merge; extend that rather than building a second merge path here.

## UI and UX Requirements

Match the application's existing design system exactly. Reuse its components,
spacing, and typography. This feature should look like it was always there.

## Responsive Requirements

Works on mobile, tablet, and desktop. Touch targets are large enough to hit on a
phone, and nothing overflows horizontally at 320px.

## Accessibility Requirements

- Fully keyboard navigable.
- Correct semantic elements and ARIA roles.
- Visible focus states.
- Meets WCAG AA contrast.
- Dynamic changes are announced to screen readers.
- Respects prefers-reduced-motion.

## Edge Cases

- A provider that returns no email, or a changed email, must still link to the right account by stable provider ID.
- An email that the provider has not verified must not satisfy any ownership check.
- Linking a provider whose email differs from the account email must not overwrite the account email.
- Unlinking a method used by an active session should not sign that session out mid-action, but the session must lose the ability to re-authenticate with it.
- Both link and unlink must be written to the audit trail and emailed to the account owner, including when the actor is an admin.
- A provider that later revokes the app's access leaves a dead identity record — detect it and prompt, rather than failing at the next sign-in.

## Testing

Exercise the feature end to end in the running application. Cover every edge case
above, then run the existing test suite and confirm nothing regressed.

## Acceptance Criteria

- [ ] One account can hold multiple sign-in methods and all of them reach the same data.
- [ ] Linking and unlinking both require recent re-authentication.
- [ ] An identity already bound to another account is refused with a clear explanation.
- [ ] The last usable sign-in method cannot be removed.
- [ ] Every link and unlink is audited and notified to the owner.
- [ ] Unverified provider emails are never used to match an account.
- [ ] The feature matches the existing design system.
- [ ] No existing functionality is broken.

## Adaptation Rules

- Match the existing design system. Do not introduce a new colour palette,
  spacing scale, or component library.
- Reuse existing components and utilities wherever they fit.
- Follow the naming, file layout, and code style already present.
- Do not upgrade, replace, or remove existing dependencies to make this
  feature fit. Adapt the feature to the app, not the app to the feature.
- Do not break existing functionality. If a change is genuinely required in
  existing code, make the smallest one that works and say so.
- If something in these instructions conflicts with how the application is
  built, follow the application and explain the deviation.

## Final Verification

Before you report the work as done:

1. Re-read the acceptance criteria above and check each one against what you
   actually built.
2. Run the application and exercise the feature end to end.
3. Run the existing test suite and confirm you have broken nothing.
4. Check the feature on mobile, tablet, and desktop widths.
5. Check keyboard navigation and focus handling.
6. Summarize what changed: files added, files modified, and anything you
   deliberately did differently because of how this application is built.

If any acceptance criterion is unmet, fix it before reporting completion.