feat: mouse scroll wheel actions

[cenonym]

Adds three mouse wheel actions, each gated by a configurable modifier using HotkeySpec.

• Scroll strip - scroll the column strip in scrolling layout
• Resize column - grow/shrink the focused column width
• Move column - shift the focused column left/right

I've tried to follow existing patterns to the best of my ability, and parse_mods_and_optional_key in hotkey.rs was made pub so the mouse wheel config can reuse the existing modifier parsing via HotkeySpec rather than duplicating it, which was my initial approach.

Have tested all three actions on a magic mouse, a janky old office mouse and the trackpad. Input is accumulated to handle continuous scrolling from mm, trackpad etc.

Mouse wheel handlers are built inside build_gesture_handlers since they share the same config reload lifecycle, but this might warrant a rename to something like build_input_handlers for clarity, unless a full separation of concerns is wanted.

A suggestion for default config was tossed in there based on how I personally use it, but obviously feel free to move this around if it feels more logical elsewhere.