Adds circular buttons with '顶' (top) and '底' (bottom) text, centered on gradient background, for scrolling to page top/bottom. On Twitter (X), top button scrolls up, shows ring animation, and refreshes timeline. On YouTube, preloads two screen heights of content ahead during any scroll for seamless scrolling, with same refresh animation. Other pages refresh with animation. Long press (300ms) to edit button positions, maintaining 60px spacing. Release to save; buttons fixed otherwise.
