draw_conversation_pane and draw_pane now delegate all scroll
bookkeeping and rendering to the ScrollPane widget. The conversation
pane builds MarkedLine items (line + gutter marker), applies
selection highlighting, and passes them to the widget. The simpler
panes just pass lines directly.
Removed dead code from scroll_pane: BorrowedItem, scroll_to_bottom,
heights(), ensure_heights_for_lines — all superseded by the widget
doing the work internally through the ScrollItem trait.
Co-Authored-By: Kent Overstreet <kent.overstreet@gmail.com>
New ScrollPaneState centralizes height caching, scroll offset,
pin-to-bottom, visible range computation, and screen-to-item
coordinate mapping. Replaces the hand-rolled scroll bookkeeping
that was duplicated across draw_conversation_pane and draw_pane.
-170 lines from chat.rs. The scroll_pane module also includes a
ScrollPane StatefulWidget ready to wire up for the next step:
collapsing the draw functions into render_stateful_widget calls.
Co-Authored-By: Kent Overstreet <kent.overstreet@gmail.com>
