feat/gestures: Differentiate workspace scrolling from page scrolling

In a Niri-style workflow:

- 3-finger gestures are used for workspace switching
- 2-finger gestures are used for scrolling

We need better differentiation between:

- scrolling inside a page/window
- scrolling between tracks/workspaces

One possible solution:
add a floating interaction box where scrolling over it switches tracks, otherwise normal page scrolling occurs.

No files, tests, or entry points are named. Start by locating the existing gesture-handling and workspace/page scrolling entry points, then define how the floating interaction box should distinguish track switching from normal scrolling. Done means two-finger gestures scroll within a page or window except over the designated box, where they switch tracks or workspaces.

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