feat(viewport): optional touchpad pan for wheel-scrolling touchpads

Outside macOS a touchpad's two-finger drag arrives as wheel scrolls that
nothing tells from a wheel. A new option (Options, off by default) makes
every scroll pan and Ctrl+scroll — what a pinch sends there — zoom.

Fixes #1560

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

src/app/update/file.rs

@@ -662,6 +662,7 @@ impl OpenCADStudio {
             polar: self.polar_mode,
             polar_increment_deg: self.polar_increment_deg,
             zoom_wheel_reversed: self.zoom_wheel_reversed,
+            touchpad_pan: self.touchpad_pan,
             zoom_factor: self.zoom_factor,
             cursor_size: self.cursor_size,
             pick_box: self.pick_box,
@@ -762,6 +763,7 @@ impl OpenCADStudio {
         self.polar_mode = s.polar;
         self.polar_increment_deg = s.polar_increment_deg;
         self.zoom_wheel_reversed = s.zoom_wheel_reversed;
+        self.touchpad_pan = s.touchpad_pan;
         self.zoom_factor = crate::app::settings::clamp_zoom_factor(s.zoom_factor);
         // Like the drafting-rotation field below, the Options field edits a
         // buffer rather than the value, so it is reseeded whenever the value