fix(scroll): Up/Down arrows now scroll transcript when composer is empty

The should_scroll_with_arrows gate was hardcoded to false since the
initial commit, which meant bare Up/Down arrows always navigated
composer history — they never scrolled the transcript view. Users in
virtual terminals (Ghostty, Codex, Kitty-protocol terminals) were
especially affected because they couldn't use the specialized Cmd+Up
/ Alt+Up shortcuts.

The gate now returns true when the composer input is empty (or
whitespace-only), so bare arrows scroll the transcript. When text
is present, arrows still navigate composer history to recall
previous prompts.

Added 3 tests covering empty, whitespace-only, and text-filled
composer states.

crates/tui/src/tui/ui.rs

@@ -7378,8 +7378,12 @@ fn is_ctrl_h_backspace(key: &KeyEvent) -> bool {
         && !key.modifiers.contains(KeyModifiers::SUPER)
 }
 
-fn should_scroll_with_arrows(_app: &App) -> bool {
-    false
+fn should_scroll_with_arrows(app: &App) -> bool {
+    // When the composer is empty (or only whitespace), Up/Down arrows
+    // scroll the transcript. When the composer has text, they navigate
+    // composer history so the user can recall previous prompts.
+    // Cmd+Up / Alt+Up always scroll regardless, handled upstream.
+    app.input.trim().is_empty()
 }
 
 fn extract_reasoning_header(text: &str) -> Option<String> {

crates/tui/src/tui/ui/tests.rs

@@ -3134,3 +3134,34 @@ fn checklist_write_renders_dedicated_card() {
         "raw JSON must NOT appear: {joined}"
     );
 }
+
+// ---- scroll_with_arrows ----
+
+#[test]
+fn scroll_with_arrows_returns_true_when_input_empty() {
+    let app = create_test_app();
+    assert!(
+        super::should_scroll_with_arrows(&app),
+        "empty composer: Up/Down should scroll transcript"
+    );
+}
+
+#[test]
+fn scroll_with_arrows_returns_true_when_input_only_whitespace() {
+    let mut app = create_test_app();
+    app.input = "   ".to_string();
+    assert!(
+        super::should_scroll_with_arrows(&app),
+        "whitespace-only composer: Up/Down should scroll transcript"
+    );
+}
+
+#[test]
+fn scroll_with_arrows_returns_false_when_input_has_text() {
+    let mut app = create_test_app();
+    app.input = "hello".to_string();
+    assert!(
+        !super::should_scroll_with_arrows(&app),
+        "text in composer: Up/Down should navigate history"
+    );
+}