Add Send Window to Main Pane Toggle Hotkey and HUD

[rgeraskin]

Add Send Window to Main Pane Toggle Hotkey and HUD

Overview

This PR introduces a new hotkey command that allows users to quickly toggle the "send new windows to main pane" setting without navigating to preferences, along with optional visual feedback via HUD notifications.

Closes #1800

This PR is built on the 'HUD System Refactor' commit from #1802

Features Added

🔥 New Hotkey Command

• Command: toggle-new-windows-to-main
• Functionality: Toggles between sending new windows to the main pane or secondary panes
• Real-time feedback: Shows current state via HUD notification (when enabled)

🎯 HUD Notification System

• Visual feedback: Displays "New Windows to Main Pane: On/Off" when toggled
• Configurable: Users can enable/disable notifications via preferences

⚙️ Preferences Integration

• New setting: "Notify when toggling new windows to main pane" checkbox in General preferences
• Default behavior: HUD notifications disabled by default to avoid overwhelming users
• KVO compliance: Proper change notifications for UI updates

Technical Implementation

Modified Files

• HotKeyManager.swift: Added hotkey registration and command handler
• WindowManager.swift: Implemented HUD display logic
• UserConfiguration.swift: Added toggle method and HUD preference support
• GeneralPreferencesViewController.xib: Added HUD notification preference checkbox
• default.amethyst: Added default configuration value
• docs/configuration-files.md: Updated documentation

Benefits

• Improved UX: Quick access to frequently-used setting without opening preferences
• Non-intrusive: HUD notifications are opt-in
• Discoverable: Listed in hotkey preferences for easy customization

This enhancement addresses user requests for quicker access to window placement controls while maintaining Amethyst's clean, non-intrusive design philosophy.