[FEAT] Implement account deactivation and change-email flows

[martian56]

Type / Priority

Feature / Medium

Summary

The account "deactivate" button does nothing, and there is no way to change your email.

Current state (evidence)

• SettingsPage.tsx deactivate button (~:1619-1621) has no onClick and no backend endpoint (handler/user.go has only GetActivity).
• The email field (~:1558-1564) is readOnly/disabled with no change-email flow.

Proposed scope

Add POST /api/users/me/deactivate/ + a confirm dialog, and an email-change request + verification flow with the matching endpoints.