fix: chat auto-follow overscroll causing bottom shake during inline stream updates

[Copilot]

When assistant output streams into an existing bottom-near message, the chat viewport could overshoot and create transient extra scrollable space, causing visible “shake” at the bottom edge. This PR tightens bottom-follow math so incremental same-line updates stay pinned without scrollbar jitter.

• Root-cause correction: bottom-follow target

  • Updated ChatPage auto-scroll to use the real max scroll offset (scrollHeight - clientHeight) instead of assigning scrollHeight directly.
  • Prevents overscroll-like behavior during rapid stream revisions near the bottom.

• Behavioral guardrail: focused regression test

  • Added a ChatPage test that simulates near-bottom streaming growth and asserts the resulting scrollTop equals the computed max offset.

// before
el.scrollTop = nextScrollHeight

// after
el.scrollTop = Math.max(nextScrollHeight - el.clientHeight, 0)