Without on_change, Flet never sends the user's typed text back to the
Python process; tf.value stays at the initial value. Fix:
- All connection/API key fields use _v dict updated by on_change lambdas;
  _save_all reads from _v instead of tf.value
- Const TextFields get on_change=lambda e: None so Flet syncs ctrl.value
- Fully tested: config.json, key files, and _write_constant round-trip