Docs: current password to change or remove the password

[SandObserver]

Summary

• Settings reference: changing the password, or turning Password Protection off, asks for the current password.
• Security: the current password is needed as well as a session, and wrong attempts count toward the login limit.
• Troubleshooting: add the three new admin messages for a wrong current password, a password changed elsewhere, and too many attempts.

Testing
npm run build and npm run check pass. Text only; message strings copied from the app's ui/i18n/en.json.

Checklist

• npm run build and npm run check both pass.
• Product UI is shown by running or capturing it, never recreated in CSS.
• Any new preview or screenshot carries no private hostnames or real data.
• Visual changes verified in both colour schemes and both docs themes.
• Links to the app repo still resolve.

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