Add Windows accessibility handler (partial #17)

[chicken602]

Adds Windows support for the accessibility handler. Covers the Windows portion of #17 — Linux is not included.

Changes

• Refactored accessibility.py into a platform-abstracted base class (AccessibilityHandlerBase) with _accessibility_mac.py and _accessibility_windows.py as subclasses
• Existing macOS behavior preserved — code moved to _accessibility_mac.py, no logic changes
• Platform detection + re-export at bottom of accessibility.py so downstream imports are unchanged
• Unsupported platforms raise OSError

Windows implementation

• UI Automation via comtypes (ElementFromPoint, GetFocusedElement, ControlViewWalker for parent info)
• UIA property extraction: Name, ControlType, AutomationId, IsEnabled, HasKeyboardFocus, etc.
• COM initialized/uninitialized per __call__ for thread safety
• _has_useful_info uses UIA-native property names and control type IDs
• comtypes added as Windows-only dep in pyproject.toml

Other fixes

• Signal handler registration wrapped in try/except ValueError for Windows thread compatibility
• DPI awareness set at startup for coordinate consistency across mss/pynput/screeninfo