style: Resolve nested scrollbars & implement auto-hiding navigation f…

[NINE-J]

This PR aims to enhance the UI reliability of the Tutorial page, which serves as the crucial first impression for new users exploring ChatDev’s features.

Previously, a conflict between the global browser scroll and internal component scroll resulted in nested scrollbars, creating a cluttered and somewhat frustrating interface. I have resolved this issue to provide a seamless, distraction-free UX that allows users to fully immerse themselves in the content.

What's Changed

• Resolved Nested Scrollbars: Readjusted the TutorialView container height to calc(100vh - 55px), effectively eliminating the conflict between internal and external scroll areas.
• Auto-hide Navigation: To maximize screen real estate, the navigation bar now smoothly slides up and hides when the user scrolls down to read content.
• Intuitive Navigation Trigger: Designed the navigation to reappear instantly yet smoothly when the user scrolls up slightly or simply hovers their mouse over the top edge of the screen.
• Enhanced UI Cohesion: Added CSS transitions to ensure the main-content area and the lang-switch container synchronize naturally with the navigation bar's movements.

How to Test

• Navigate to the /tutorial page.
• Verify that only a single, clean custom scrollbar is rendered on the right.
• Scroll down and confirm the top navigation bar (Home / Tutorial / Workflows...) slides up and out of view.
• Scroll up slightly or move the mouse cursor to the very top edge of the browser viewport to trigger the navigation bar's return.
• Check that the language selection buttons (English / 中文) in the top-right corner follow the navigation movement smoothly.