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/command.rs

@@ -431,6 +431,10 @@ pub(super) fn on_tab_close(&mut self, idx: usize) -> Task<Message> {
     }
 
     pub(super) fn on_command_escape(&mut self) -> Task<Message> {
+                // Esc drops an unconsumed one-shot snap override and closes
+                // its menu (#337). Falls through — Esc keeps its usual effect.
+                self.snap_override_popup = None;
+                self.snapper.clear_override();
                 // Open MText editor swallows Escape (cancel without committing).
                 if self.mtext_editor.is_some() {
                     self.mtext_cancel();