fix(settings): cover SnapType::ObjectPick in persisted-mode match

PR #80 added the internal SnapType::ObjectPick variant, which left the
exhaustive snap_id match in the settings module non-exhaustive and broke
the build after the merge. Map it to a stable id; it is not a
user-toggleable OSNAP mode, so it never enters SNAP_ORDER or the
persisted set.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/settings.rs

@@ -46,6 +46,9 @@ fn snap_id(s: SnapType) -> &'static str {
         SnapType::ApparentIntersection => "apparentintersection",
         SnapType::Parallel => "parallel",
         SnapType::Grid => "grid",
+        // Internal plugin object-pick snap — not a user-toggleable OSNAP mode,
+        // so it never enters SNAP_ORDER / the persisted set.
+        SnapType::ObjectPick => "objectpick",
     }
 }