Launching with nothing usable on the board no longer dead-ends: when
neither mode can be derived (nothing selected, no marker rectangles,
no config), the dialog now opens anyway - load-only - instead of
failing with an error figure.

- Both mode radios are unchecked and disabled with their reasons
  shown; OK and Save config... are disabled too. Load config... (and
  Cancel) stay live, so a saved config can bootstrap the run without
  first selecting pads or drawing marker rectangles.
- A loaded file re-derives everything exactly as if it had been
  present at launch - mode, net, terminals, values.