Edit remote: form doesn't pre-fill existing values and forces password re-entry (blank may overwrite stored secret)

[nuschpl]

Environment: rclone-webui-react via rclone rcd --rc-web-gui, rclone v1.74.0-DEV, browser Firefox. Affects any backend with Required and/or IsPassword options.

Steps to reproduce

Create a remote that has required fields including a password (IsPassword) option.
Click Update to edit it.
On the options step, the account-name and password fields are empty and flagged required (red), even though values are already stored.

Expected

Non-secret fields (e.g. username) pre-filled from the existing config (config/get).
The password field may stay blank for security, but leaving it blank on submit should keep the existing secret — config/update only updates the keys actually sent.

Actual

All fields appear empty/required; the user must re-type the password to save any change, and submitting a blank password risks overwriting the stored secret.

Proposed fix

Prefill the edit form from config/get (mask/omit IsPassword values).
On update, send only changed fields and omit a blank password so the existing secret is preserved.
Possibly related: #133.

Filed as: TBD — paste URL after filing