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/locale_catalog.rs

@@ -4797,6 +4797,7 @@ pub(super) fn message_attribute(source: &str) -> Option<(&'static str, &'static
         "User Preferences" => Some(("common", "user-preferences")),
         "Zoom" => Some(("common", "zoom")),
         "Reverse mouse wheel zoom (ZOOMWHEEL)" => Some(("common", "reverse-mouse-wheel-zoom-zoomwheel")),
+        "Touchpad: scroll pans, Ctrl+scroll or pinch zooms" => Some(("common", "touchpad-scroll-pans-ctrl-scroll-or-pinch-zooms")),
         "Zoom factor" => Some(("common", "zoom-factor")),
         "How far one wheel notch zooms (ZOOMFACTOR)." => Some(("common", "how-far-one-wheel-notch-zooms-zoomfactor")),
         "Text and Dimensions" => Some(("common", "text-and-dimensions")),