Refactor showStatsWindow, showInventoryWindow, and showChatWindow to use
a shared createWindow() helper that handles existence checks, z-index
management, header/close button creation, and makeDraggable setup. Each
function now only contains its unique content creation logic.
Added .window-content CSS class to style.css, style-ac.css, and the
christmas theme to maintain flex layout through the new wrapper div.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
