input: implement follow_mouse_shrink (#13707)

* input: implement follow_mouse_shrink

Add a new config option `input:follow_mouse_shrink` (INT, 0-300, default 0)
that shrinks inactive window hitboxes by the specified number of pixels for
focus detection purposes. This creates a dead zone in gaps between windows
where moving the cursor will not trigger a focus change.

The shrink only applies to inactive (non-focused) windows and only during
mouse-move focus checks (follow_mouse = 1). Click and scroll interactions
are unaffected — clicking on a window in the shrunk area still works
normally once the window is focused.

Closes #9973

* tests: add integration test for follow_mouse_shrink

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