Release 1.4.2: load-only dialog when nothing is selected

With neither mode derivable (nothing selected, no marker rectangles,
no config) the dialog now opens anyway instead of failing with an
error figure: both radios disabled with their reasons, OK/Save
disabled, Load config… live - a saved config can bootstrap the run.

README: note that macOS/NixOS field tests last ran with plugin v1.3.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HjSRLWQ8ywBBYyakvr3YTx

docs/release-notes/v1.4.2.md

@@ -0,0 +1,11 @@
+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.