fix: reauthenticate only for sensitive settings changes

[darwin67]

Summary

• allow authenticated users to open settings and update non-sensitive paste defaults without reauthenticating
• require recent authentication only when changing email/password or managing API tokens, with safe redirects for stale sessions
• serialize email confirmations, bind tokens to their user, and atomically consume them
• handle invalid direct password updates without crashing and re-login using the server-returned user

Regression coverage

• stale settings access and sensitive/non-sensitive mutation boundaries
• cross-user and concurrent email-confirmation attempts
• invalid and identity-field-free direct password submissions

Verification

• nix develop -c mix precommit (454 tests passed)