fix(input): forward vertical mouse wheel to GUI

[kakra]

Summary

Forward vertical mouse-wheel input from the XCB and Xlib backends to ImGui.

• translate X11 button 4/5 presses into vertical wheel deltas
• accumulate wheel input in InputManager
• consume the accumulated delta once per GUI frame
• observe Xlib wheel events through a separate display connection without consuming events from the application's queue
• leave native Wayland axis normalization for a separately testable follow-up

Horizontal scrolling is intentionally outside the scope of this change.

Testing

• release build with meson compile -C build
• exercised through the vkShade GUI under ETS2 via Proton/XWayland

[kakra]

Converting back to draft to add more work:

• horizontal mouse wheel support (I can test this here if the GUI has horizontal scroll areas)
• Wayland support

[ralgar]

Closing this now since it's superseded by #89