feat(select): PICKADD/PICKDRAG command-line toggles (#226)

PICKADD 0 = click/box/lasso replaces the selection (Shift toggles),
empty click clears; PICKADD 1 (default) keeps today's accumulate.
PICKDRAG 1 = press-drag spans a rectangle marquee through the box
machinery; PICKDRAG 0 (default) keeps the freeform lasso. Both persist
in settings.json. Also re-points the start-page gate test at the
start_allowed fn it moved into (#388/389 refactor left it broken).

Closes #226

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/app/commands/mod.rs

@@ -290,6 +290,8 @@ inventory::submit!(crate::command::CommandRegistration {
         // Restore the last-erased objects.
         "OOPS",
         // Plan (top) view.
+        "PICKADD",
+        "PICKDRAG",
         "PLAN",
         // Current object colour (CECOLOR).
         "COLOR",