The Send User-Agent switch was only derived from settingsConfig.headers
on mount and on preset reset. Editing headers directly in the
Configuration JSON editor left it stale in both directions: adding a UA
in JSON kept the switch off (and clicking it then replaced the custom
value with the default UA), removing it kept the switch on.
Add syncOpenclawUserAgentFromConfig to useOpenclawFormState and call it
from the OpenClaw JSON editor's onChange. It only reads valid JSON
objects, sets the switch to the case-insensitive presence of a
User-Agent header, never rewrites the JSON, and keeps the last state
while the draft is invalid.
Refs #8256