Story 22: Implement smart auto-scroll that respects user scrolling

User Story:
As a user, I want to be able to scroll up to review previous messages
while the AI is streaming or adding new content, without being
constantly dragged back to the bottom.

Implementation:
- Replaced position-based threshold detection with user-intent tracking
- Detects when user scrolls UP and disables auto-scroll completely
- Auto-scroll only re-enables when user manually returns to bottom (<5px)
- Uses refs to track scroll position and direction for smooth operation
- Works seamlessly during rapid token streaming and tool execution

Technical Details:
- lastScrollTopRef: Tracks previous scroll position to detect direction
- userScrolledUpRef: Flag set when upward scrolling is detected
- Direct scrollTop manipulation for instant, non-fighting scroll behavior
- Threshold of 5px from absolute bottom to re-enable auto-scroll

Spec Updates:
- Added comprehensive Smart Auto-Scroll section to UI_UX.md
- Documented the problem, solution, requirements, and implementation
- Includes code examples and edge case handling

Acceptance Criteria Met:
✅ Auto-scroll disabled when scrolling up
✅ Auto-scroll resumes when returning to bottom
✅ Works normally when already at bottom
✅ Smooth detection without flickering
✅ Works during streaming and tool execution

Files Changed:
- src/components/Chat.tsx: Implemented user-intent tracking
- .living_spec/specs/functional/UI_UX.md: Added Smart Auto-Scroll spec
- .living_spec/stories/22_smart_autoscroll.md: Marked complete

.living_spec/stories/22_smart_autoscroll.md

@@ -4,13 +4,13 @@
 As a user, I want to be able to scroll up to review previous messages while the AI is streaming or adding new content, without being constantly dragged back to the bottom.
 
 ## Acceptance Criteria
-- [ ] When I scroll up in the chat, auto-scroll is temporarily disabled
-- [ ] Auto-scroll resumes when I scroll back to (or near) the bottom
+- [x] When I scroll up in the chat, auto-scroll is temporarily disabled
+- [x] Auto-scroll resumes when I scroll back to (or near) the bottom
 - [ ] There's a visual indicator when auto-scroll is paused (optional)
 - [ ] Clicking a "Jump to Bottom" button (if added) re-enables auto-scroll
-- [ ] Auto-scroll works normally when I'm already at the bottom
-- [ ] The detection works smoothly without flickering
-- [ ] Works during both streaming responses and tool execution
+- [x] Auto-scroll works normally when I'm already at the bottom
+- [x] The detection works smoothly without flickering
+- [x] Works during both streaming responses and tool execution
 
 ## Out of Scope
 - Manual scroll position restoration after page refresh