Fix scroll: PgUp/PgDn move cursor in place, add Scrollbar widget

SectionTree.handle_nav() now takes viewport height:
- PgUp/PgDn move both cursor and viewport by one page, keeping the
  cursor at the same screen position
- Home/End jump to first/last item
- scroll_to_selected() uses actual viewport height instead of
  hardcoded 30

Added render_scrollable() in widgets.rs: renders a Paragraph with a
vertical Scrollbar when content exceeds the viewport. Used by the
conscious and subconscious screens.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/user/context.rs

@@ -7,12 +7,11 @@ use ratatui::{
     layout::Rect,
     style::{Color, Style},
     text::Line,
-    widgets::{Paragraph, Wrap},
     Frame,
 };
 
 use super::{App, ScreenView, screen_legend};
-use super::widgets::{SectionTree, pane_block};
+use super::widgets::{SectionTree, pane_block, render_scrollable};
 
 pub(crate) struct ConsciousScreen {
     agent: std::sync::Arc<tokio::sync::Mutex<crate::agent::Agent>>,
@@ -41,7 +40,7 @@ impl ScreenView for ConsciousScreen {
             if let ratatui::crossterm::event::Event::Key(key) = event {
                 if key.kind != ratatui::crossterm::event::KeyEventKind::Press { continue; }
                 let context_state = self.read_context_state();
-                self.tree.handle_nav(key.code, &context_state);
+                self.tree.handle_nav(key.code, &context_state, area.height);
             }
         }
 
@@ -97,11 +96,6 @@ impl ScreenView for ConsciousScreen {
         let block = pane_block("context")
             .title_top(Line::from(screen_legend()).left_aligned());
 
-        let para = Paragraph::new(lines)
-            .block(block)
-            .wrap(Wrap { trim: false })
-            .scroll((self.tree.scroll, 0));
-
-        frame.render_widget(para, area);
+        render_scrollable(frame, area, lines, block, self.tree.scroll);
     }
 }