scroll_pane: extract scroll state from chat.rs

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>

src/user/mod.rs

@@ -5,6 +5,7 @@
 
 pub(crate) mod chat;
 mod context;
+pub(crate) mod scroll_pane;
 mod subconscious;
 mod unconscious;
 mod thalamus;