Ctrl+left drag on empty space orbits in every navigation preset (#471)

A left press with Ctrl (or Cmd) held, no Shift or Alt, on empty space orbits
for that press, unless the preset binds Ctrl+left itself. It only orbits
once the pointer moves: a Ctrl+click still adds to the selection, and
Ctrl+drag on sketch geometry still drags it without snapping. Works with a
tool active, without adding geometry. Ctrl+box selection from empty space
becomes the orbit; Shift+box selection still adds.

Preferences › Navigation and the UI tour mention it. Test drives every
preset through real pointer events.


Claude-Session: https://claude.ai/code/session_01YDDPmNdRoBgkHcLeUDKVfb

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/ui-egui/src/prefs.rs

@@ -816,7 +816,7 @@ fn page(app: &mut SolveApp, ui: &mut egui::Ui, t: &Tokens) {
             });
             ui.end_row();
             let snap_help = if app.preferences.nav == OPENSCAD_NAV { "" } else { "; hold Shift during orbit to snap" };
-            note(ui, &format!("{}{snap_help}; the wheel zooms", nav_preset(&app.preferences.nav).help));
+            note(ui, &format!("{}; Ctrl+left drag on empty space orbits{snap_help}; the wheel zooms", nav_preset(&app.preferences.nav).help));
             ui.label("Context menu");
             ui.checkbox(&mut app.preferences.radial_context_menu, "Radial context menu");
             ui.end_row();