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 <[email protected]>

src/app/view/modal.rs

@@ -316,6 +316,7 @@ impl OpenCADStudio {
                             cliprompt_lines: self.cliprompt_lines,
                             commandline_fade_ms: self.commandline_fade_ms,
                             zoom_wheel_reversed: self.zoom_wheel_reversed,
+                            touchpad_pan: self.touchpad_pan,
                             zoom_factor: self.zoom_factor,
                             texteditmode: self.texteditmode,
                             dimension_continue_mode: self.dimension_continue_mode,