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/update/mod.rs

@@ -605,6 +605,7 @@ impl OpenCADStudio {
         match msg {
             Message::SpaceMouseWake => self.on_spacemouse_wake(),
             Message::TrackpadPinch(magnification) => self.on_pinch_zoom(magnification),
+            Message::Touch(event) => self.on_touch(event),
             Message::SpaceMouseFrame(time) => {
                 self.spacemouse
                     .frame(time.saturating_duration_since(self.start).as_secs_f64() * 1000.);
@@ -4343,6 +4344,18 @@ impl OpenCADStudio {
             // ── Cursor / viewport messages ─────────────────────────────────
             Message::CursorMoved(p, viewport) => self.on_cursor_moved(p, viewport),
 
+            // A multi-finger gesture owns the pointer while it runs.
+            Message::ViewportMove(_)
+            | Message::ViewportLeftPress
+            | Message::ViewportLeftRelease
+            | Message::PaneMove(..)
+            | Message::PanePress(_)
+            | Message::PaneRelease(_)
+                if self.touch.active() =>
+            {
+                Task::none()
+            }
+
             Message::ViewportMove(p) => self.on_viewport_move(p),
 
             Message::ViewportExit => self.on_viewport_exit(),