Add Touchpad navigation: Shift to pan, Alt/Option to orbit (#1220)

An opt-in Touchpad navigation preset: Shift + pointer motion pans, Alt/Option + motion orbits, vertical scroll zooms, all without a mouse button. Existing presets are unchanged; view.navigate gains continuous motion for the control channel and scenarios.

crates/ui-egui/src/canvas.rs

@@ -197,6 +197,13 @@ fn edge_bisector(m: &Mesh, index: usize, p: Vec3) -> Option<Vec3> {
     sum.normalized()
 }
 
+/// Whether a dynamically registered canvas handle owns this point before navigation.
+pub(crate) fn owns_pointer(app: &SolveApp, proj: &Proj, p: Pos2) -> bool {
+    // The triad only registers an egui hit widget after a handle is under the pointer.
+    // Test its geometry before navigation, including the first frame entering a handle.
+    app.dialog.as_ref().and_then(|d| crate::dialogs_move::triad(app, d)).is_some_and(|t| crate::gizmo::contains_pointer(proj, &t, p))
+}
+
 /// Draw the manipulator and the value box (called by the viewport after the model is drawn).
 pub fn show(app: &mut SolveApp, ui: &mut egui::Ui, painter: &egui::Painter, proj: &Proj) {
     // While OK computes, the inputs hold what it computes: no arrow to drag, no box to type in.