fix(ui): use cursor position for scroll-intent tracking

[jensenojs]

• output_window: is_at_bottom() checks cursor instead of viewport (viewport almost always at bottom in streaming chat)
• CursorMoved autocmd replaces WinScrolled for both windows
• state: add cursor persistence functions (save/get/set)
• output_window: add buffer_valid() guard for buffer-only operations

[jensenojs]

for

I find one issue where if you move the cursor in the outpout window withoud scrolling and close/reopen, the cursor will be set at the bottom instead of restoring to the last position.

• Currently splitting implementation via new PRs - feat(ui): preserve buffers on toggle #229 (comment)

[sudo-tee]

Thanks,

This one make sense and is pretty straight forward