Add UI for passwordless passkey-only accounts

[devondragon]

Summary

Once the library supports passwordless passkey-only accounts (devondragon/SpringUserFramework#254), add UI support in the demo app.

UI Changes Needed

• Profile page: Option to remove password when user has at least one passkey registered
• Auth method indicator: Visual indicator showing account auth method (password, passkey, or both)
• Confirmation dialog: Warning when removing password, explaining that passkeys will be the sole auth method
• Registration flow: If passkey-only signup is supported in the library, update the registration page to offer it

Depends On

• Support passwordless passkey-only accounts SpringUserFramework#254