Add keyboard shortcut to move current tab to new window

[Copilot]

Adds a keyboard shortcut (Ctrl+Shift+N / Cmd+Shift+N) to move the current tab to a new window, addressing the functionality gap where this action was only available via the context menu "move tab > move to new window" option.

Changes

• New Command: cmd_zenMoveTabToNewWindow that safely moves the current tab to a new window
• Keyboard Shortcut: Ctrl+Shift+N (Windows/Linux) or Cmd+Shift+N (Mac) with automatic migration for existing users
• Safety Checks: Prevents execution when only one tab is open or when on an empty tab
• Preferences Integration: Appears under "Window and Tab Management" in keyboard shortcuts preferences

Technical Implementation

The implementation leverages Firefox's existing gBrowser.replaceTabWithWindow() method for reliability and follows all established Zen browser patterns:

case 'cmd_zenMoveTabToNewWindow': {
  const currentTab = gBrowser.selectedTab;
  if (currentTab && !currentTab.hasAttribute('zen-empty-tab') && gBrowser.tabs.length > 1) {
    gBrowser.replaceTabWithWindow(currentTab);
  }
  break;
}

The keyboard shortcut is automatically added to existing installations via migration version 12, ensuring all users can benefit from this feature without manual configuration.

User Experience

Users can now quickly move tabs to new windows using a keyboard shortcut, improving workflow efficiency for power users who prefer keyboard navigation over context menus. The shortcut is discoverable and customizable through the standard preferences interface.

Fixes functionality request where users wanted keyboard access to the existing "move to new window" feature that was previously only accessible via right-click context menu.

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