Chat scrolls itself only when you are at the bottom, otherwise a 'new messages' pill appears; system lines are muted #286

From #51.

Done:

• [ ] A new line auto-scrolls the list only when it was already near the bottom. Otherwise a "new messages" pill appears, and tapping it jumps to the bottom.
• [ ] The list has overscroll-behavior: contain. System events ("X is driving") are styled muted.
• [ ] Tests: scrolled up, a new line shows the pill and keeps the position; at the bottom, it scrolls.
• [ ] forge.json untouched. bun --cwd Software/so101 test passes.