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/input/mod.rs

@@ -1,3 +1,4 @@
 //! Device integrations. Camera ownership and CAD commands stay in the app.
 pub(crate) mod spacemouse;
+pub(crate) mod touch;
 pub(crate) mod trackpad;