bug(auth-client): fix "Incorrect email case" on password checks

[nshirley]

Because

• When an account's signup email (accounts.email) differs from its current
  primary email, the current-password check derived v1 credentials from the
  signup email — but the stored verifier may be salted with the primary email
  (e.g. after a password reset that followed a primary-email change). The
  mismatch surfaced to users as an "Incorrect email case" error, blocking
  Change Password, Create Recovery Key, and Delete Account with no self-serve
  workaround.

This pull request

• Salts the first current-password derivation attempt with the primary email
  (the address the user authenticates with) and falls back to the signup email
  on INCORRECT_EMAIL_CASE, in sessionReauth, accountDestroy, and
  passwordChangeWithJWT (packages/fxa-auth-client/lib/client.ts). Accounts
  where the two emails match are unaffected (single attempt).
• Fixes a latent retry bug in accountDestroy where the retry spread the
  server error object into the email input, resolving the payload email to the
  signup address instead of the primary.
• Adds unit tests in packages/fxa-auth-client/test/client.ts covering the
  primary→signup fallback, the single-request happy path, and non-email-case
  errors not retrying.

Issue that this pull request solves

Closes: FXA-14223

Checklist

Put an x in the boxes that apply

• My commit is GPG signed.
• If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).
• I have manually reviewed all AI generated code.

How to review (Optional)

• Key files/areas to focus on: the salt selection in sessionReauth /
  accountDestroy (conditional) and passwordChangeWithJWT (candidate loop).
• Suggested review order: sessionReauth first (shared pattern), then
  passwordChangeWithJWT, then the tests.
• Risky or complex parts: the salt is intentionally decoupled from each route's
  identity email — reauth identifies the account by the primary, while the
  /mfa/password/change payload keeps the signup email so the server's
  emailsMatch(email, accounts.email) check still holds.

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Other information (Optional)

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