fix - chat not scrolling to the newest messages in long conversations

[xinyi-gong]

Summary

Fixes #63. In long conversations the chat could no longer be scrolled down
to the latest messages: the scrollbar looked scrolled to the bottom (or kept
moving) but the content stopped, leaving the newest turns unreachable.

Root cause

ChatContentViewer extended ScrolledComposite, which scrolls by moving a
single content child whose height equals the entire conversation. On Windows
a native child window position is a signed 16-bit value, so once the required
offset exceeded that range the OS clamped it — the content could not move
further and the bottom became unreachable. This matches every reported
symptom: only long sessions, Windows only, and "maximize the view" or
"Ctrl+-" (which shrink the total pixel height) working around it.

Fix

Rewrite ChatContentViewer as a self-managed, windowed vertical scroller
(extends Composite with SWT.V_SCROLL):

• cmpContent is pinned to the viewport rectangle and never grown or moved.
• A 32-bit logical model (scrollOffset / totalHeight / per-turn tops)
  drives scrolling; each turn is positioned at y = top - scrollOffset.
• Turns intersecting the viewport are laid out; the rest are parked with
  setVisible(false), so no native child ever receives an out-of-range
  coordinate.

Behavior preserved from the previous implementation:

• Auto-scroll: scrolling away from the bottom pauses follow-the-tail
  during streaming; scrolling back to the bottom resumes it.
• Latest user turn pinned to the top: reserved via virtual bottom
  padding, which also keeps "scroll to bottom" aligned with the real maximum
  so auto-scroll stays correct for both short and long rounds.
• Streaming stays cheap: incremental passes re-measure only the trailing
  (mutating) turns; sealed turns keep cached heights.

Scroll step now derives from the OS-provided wheel line count (event.count)
times the font line height, so it follows the user's system scroll settings
and DPI instead of a fixed pixel constant.

[Copilot]

Pull request overview

This PR fixes a Windows-only scrolling failure in long chat conversations by replacing ScrolledComposite-based scrolling (which can hit native coordinate limits) with a self-managed, windowed vertical scroller in ChatContentViewer.

Changes:

• Reworked ChatContentViewer to extends Composite with a logical scroll model (scrollOffset, totalHeight) and manual child positioning.
• Added windowing + measurement caching (IdentityHashMap height cache) to avoid remeasuring sealed turns and to keep off-viewport turns hidden.
• Implemented explicit mouse wheel scrolling and scrollbar synchronization for the manual layout approach.

[jdneo]

LGTM