feat(macos): implement mouse button input and side button output support

[FlameFlag]

Partially addresses #1470 (mapping mouse buttons on macOS)

Describe your changes. Use imperative present tense

Add mouse button input interception and side button (forward/backward) output
support on macOS

Mouse button output (side buttons)

Fix mbck and mfwd actions which currently panic on macOS because the code
does not know which CGEventType to use for side buttons

Use CGEventType::OtherMouseDown/Up (same event type as middle click) and
override the MOUSE_EVENT_BUTTON_NUMBER field to the correct button index (3
for Back, 4 for Forward). The Rust core-graphics crate's CGMouseButton enum
only has 3 variants but the underlying Apple API supports up to 32 buttons via
this field

Ref: init(mouseEventSource:mouseType:mouseCursorPosition:mouseButton:), setIntegerValueField

Mouse button input (event tap)

Before this change, mouse buttons in defsrc had no effect on macOS because the
Karabiner DriverKit only captures keyboard HID events

Add a CGEventTap on a background thread that intercepts mouse button
events at the HID level. Suppress mapped buttons and forward them to kanata's
processing channel for remapping; pass unmapped buttons through. Follow the same
architecture as the Windows mouse hook (src/kanata/windows/llhook.rs). Only
install the tap if the config has mouse buttons in defsrc

Require Accessibility or Input Monitoring permission in System Settings >
Privacy & Security

Checklist

• Add documentation to docs/config.adoc
  • Yes (updated mouse button section to include macOS, added permission note)
• Add example and basic docs to cfg_samples/kanata.kbd
  • N/A (mouse button examples already exist in kanata.kbd lines 1044-1072; no new actions added)
• Update error messages
  • Yes (error log when event tap creation fails with permission guidance)
• Added tests, or did manual testing
  • Yes, Manual testing on macOS with a USB mouse (side buttons, middle click, left, right)

[jtroo]

Thanks!