[Mouse Highlighter] Add shortcut to switch between highlight modes

[Copilot]

This PR adds a new configurable shortcut to switch between spotlight and circle highlight modes in Mouse Highlighter, allowing users to quickly change modes without opening the settings panel.

Changes Made

Backend Implementation (C++)

• Added MouseHighlighterSwitchMode() API function that toggles the spotlight mode setting
• Implemented WM_SWITCH_HIGHLIGHT_MODE message handling to restart drawing with the new mode
• Updated the module interface from single hotkey (GetHotkeyEx) to multiple hotkeys (get_hotkeys) to support both activation and mode switch shortcuts
• Added proper parsing for the new mode switch shortcut in settings configuration

Settings & UI (C#)

• Added ModeSwitchShortcut property to MouseHighlighterProperties.cs with default Win+Shift+M binding
• Updated MouseUtilsViewModel.cs to expose the new shortcut property and include it in hotkey conflict detection
• Added UI control in MouseUtilsPage.xaml for configuring the mode switch shortcut
• Added localization strings for the new feature

User Experience

Before: Users had to open PowerToys Settings → Mouse utilities → Mouse Highlighter and toggle the "Spotlight mode" checkbox to switch between highlighting modes.

After: Users can now:

• Use Win+Shift+H to toggle Mouse Highlighter on/off (existing functionality)
• Use Win+Shift+M to instantly switch between Circle and Spotlight modes (new functionality)
• Configure both shortcuts independently in the settings UI
• Switch modes only when Mouse Highlighter is active

Technical Details

The implementation follows established PowerToys patterns for multi-hotkey modules and maintains backward compatibility. The mode switching works by:

1. Detecting the hotkey via the centralized keyboard hook
2. Calling the new MouseHighlighterSwitchMode() function
3. Toggling the internal m_spotlightMode flag
4. Restarting the drawing system with the new mode if currently active

This provides instant visual feedback when switching between circle highlights (for mouse clicks) and spotlight mode (continuous mouse following).

Fixes #41565.

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