refactor(core): move gesture mode into config/gestures.rs

Gesture mode is the one part of `impl Config` with rules of its own: reading the mode off the binding shape, promoting a button through the private `ensure_gesture_binding`, and the stash in `DeviceConfig::disabled_gestures` that makes an off/on round trip lossless. Those five methods sat between the keyboard accessors and the per-app overlay, 126 lines that none of their neighbours call. They move verbatim into `config/gestures.rs`; `ensure_gesture_binding` stays private, since both of its callers move with it. `config.rs` no longer names `GestureDirection` or the three `default_*` helpers itself, so they leave its imports; `tests.rs` still reaches `GestureDirection` through `use super::*`, which the parent now feeds with a test-only import, as it already does for the two backup helpers.