feat: add stylus barrel button action bindings

[hakan-demirli]

Summary

• Add configurable action bindings for stylus barrel buttons (BTN_STYLUS / 331 and BTN_STYLUS2 / 332)
• Handle zwp_tablet_tool_v2::Event::Button events and dispatch the configured action on press
• Default: primary barrel button (331) opens the radial menu; secondary (332) is unbound
• Existing proximity-based eraser auto-switch is unaffected

Details

Adds a StylusButtonBinding config type with a single action field that maps to any keybinding action. The binding is looked up and dispatched directly on button press without interacting with tool_override or the eraser auto-switch path.

Barrel buttons that trigger physical tool-type changes (pen/eraser cycling) continue to work through the existing auto_eraser_switch proximity handler.

Config

[tablet.stylus_button]
action = "toggle_radial_menu"

[tablet.stylus_button2]
# action = "undo"

[devmobasa]

Thanks @hakan-demirli