feat(snap): Shift+RMB one-shot snap override menu (#337)

Shift+right-click in the viewport opens a cursor-anchored grid of snap
icons (names as hover tooltips). Picking one engages a one-shot
override: only that snap applies — even with running osnap off — until
the next point pick commits, which restores the saved configuration.
Esc drops an unconsumed override; an outside click closes the menu.
The status-bar snap toggles are untouched — this is a separate,
temporary mechanism.

Covers items 1 and 2 of #337 (mid-between-two-points still pending).

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

src/app/update/viewport.rs

@@ -2175,6 +2175,9 @@ pub(super) fn on_tick(&mut self, t: Instant) -> Task<Message> {
                             }
                         }
                         self.last_point = Some(world_pt);
+                        // The one-shot snap override is spent by this pick —
+                        // restore the running osnap configuration (#337).
+                        self.snapper.clear_override();
                         self.dyn_user_reshaped = false;
                         self.sync_dyn_fields();
                         self.reset_tracking_after_point();