Add j/k shortcuts for scrolling and update help instructions in TUI

fixes #18

crates/owlen-tui/src/ui.rs

@@ -849,6 +849,8 @@ fn render_help(frame: &mut Frame<'_>) {
         Line::from("  i / Enter   → start typing"),
         Line::from("  Enter       → send message"),
         Line::from("  Ctrl+J      → newline"),
+        Line::from("  j / ↓       → scroll down"),
+        Line::from("  k / ↑       → scroll up"),
         Line::from("  m           → select model"),
         Line::from("  n           → new conversation"),
         Line::from("  c           → clear conversation"),