web-ui: fix select value binding and add a default effort picker (#567)

- fieldSelect binds .value through live() and options set ?selected, so
  a <select> shows its saved value on first render and re-renders can't
  silently reset it
- the scope model panel holds an in-flight pick until the save settles
  (no visual snap-back during the round-trip; reverts on error)
- when a model is pinned, the panel offers a Default effort dropdown,
  filtered to the harness's supported levels and carried across model
  switches when still valid

Co-authored-by: qm <qm@ycombinator.com>