Settings/Billing Page

PROJECT CONTEXT:
I am adding an account settings page where users manage profile, password, plan, and account deletion.

GOAL:
Implement a settings page whose every control actually persists: profile, password, preferences, plan, and delete account.

REQUIREMENTS:
- Profile section (name, email) with validated persistent edits
- Password change requiring the current password
- Preferences (e.g. notifications toggle) that persist
- Plan/billing section showing the current plan
- Danger zone: delete account with confirmation

CONSTRAINTS:
- Every toggle and save must persist - no decorative controls
- Password change must verify the current password
- Account deletion must actually remove access

DEFINITION OF DONE:
- Profile edits survive refresh and re-login
- Password change works and old password stops working
- Preference toggles persist
- Delete account logs out and blocks re-login

COMMON FAILURES TO AVOID:
- Save buttons that show a toast but write nothing
- Password change that does not verify the current password
- Toggles resetting on reload
- Delete account that only redirects home