Resolves a request to navigate the Canvas with a regular mouse: - Cmd + scroll wheel zooms around the cursor (anchor-preserving), reusing the same math as the existing pinch gesture. - Middle-mouse-button drag pans the canvas regardless of which terminal currently has focus, via a window-scoped local NSEvent monitor that swallows the events so they never reach Ghostty. The monitor is installed/torn down with the canvas view's window membership, so it has no effect outside Canvas mode. Closes #197