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

fill_resistance/main.py

@@ -7,7 +7,10 @@ config's terminal set) -> gather fills -> dialog with a Classic/PDN
 mode selector (classic: the two-contact form; PDN: editable per-role
 terminal tables) -> extract vias -> solve -> figures + report. The
 dialog's "Load config…" button loops back to the derivation with the
-picked file, so a run can be set up from any saved config.
+picked file, so a run can be set up from any saved config - and the
+dialog opens even when NEITHER mode is derivable (nothing selected, no
+marker rectangles, no config): load-only, both radios disabled with
+their reasons, so a saved config can bootstrap the run.
 
 Every failure is reported twice: on stdout (lands in the KiCad status-bar
 warning list) and as a matplotlib error figure, so it cannot be missed.
@@ -101,8 +104,10 @@ def main() -> None:
                     configfile.apply_physics(cfg)
 
                 # BOTH terminal derivations always run; a failure only
-                # disables that mode's radio (with the reason shown) - the
-                # launch dies only when neither mode is possible
+                # disables that mode's radio (with the reason shown).
+                # Even with neither mode possible the dialog still
+                # opens - load-only - so "Load config…" works without
+                # first selecting pads or drawing marker rectangles
                 classic_reason = pdn_reason = None
                 es1: list = []
                 es2: list = []
@@ -210,9 +215,13 @@ def main() -> None:
                               f"workable supply+load copper; PDN "
                               f"candidates: {', '.join(pdn_nets)}")
                 if classic_reason is not None and pdn_reason is not None:
-                    raise SelectionError(
-                        f"{classic_reason}\n(PDN mode is also "
-                        f"unavailable: {pdn_reason})")
+                    # nothing runnable right now: the dialog opens with
+                    # both radios and OK/Save disabled, offering only
+                    # Load config… (and Cancel) - a loaded file then
+                    # re-derives everything through the loop below
+                    print(f"note: classic mode unavailable: "
+                          f"{classic_reason}")
+                    print(f"note: PDN mode unavailable: {pdn_reason}")
                 buildups = board_io.gather_mask_buildups(board)
             except ApiError as e:
                 raise UserFacingError(