feat(input): touch screen gestures on the drawing

One finger already worked as the mouse. Two or more fingers now make a
gesture instead of fighting over the cursor: two drag to pan and pinch to
zoom about the fingers, three drag to orbit; a quick two-finger tap opens
the context menu, a three-finger tap zooms to the extents and a quick
three-finger swipe up shows the isometric view. While a gesture runs the
drawing area's own pointer handling keeps out, and the press the first
finger started is dropped.

Refs #906

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/view/mod.rs

@@ -2717,6 +2717,7 @@ impl OpenCADStudio {
                 .map(|_| Message::SpaceMouseWake),
             crate::input::trackpad::subscription().map(Message::TrackpadPinch),
             event::listen_with(|event, _, id| match event {
+                iced::Event::Touch(touch) => Some(Message::Touch(touch)),
                 iced::Event::Window(window::Event::Focused) => {
                     Some(Message::SpaceMouseFocus(id, true))
                 }