fix(tui): wrap color backward cycle at index 0 instead of clamping (closes #29)

crates/tui/src/app.rs

@@ -757,7 +757,7 @@ impl App {
                 if forward {
                     (p + 1) % colors.len()
                 } else {
-                    p.saturating_sub(1).min(colors.len() - 1)
+                    if p == 0 { colors.len() - 1 } else { p - 1 }
                 }
             }
             None => {
@@ -1936,6 +1936,14 @@ plain text line
 
         app.handle_key(make_key(crossterm::event::KeyCode::Left));
         assert_eq!(app.settings_draft.error, "red");
+
+        // backward wrap: red (index 0) → white (index 7)
+        app.handle_key(make_key(crossterm::event::KeyCode::Left));
+        assert_eq!(app.settings_draft.error, "white");
+
+        // forward wrap: white (index 7) → red (index 0)
+        app.handle_key(make_key(crossterm::event::KeyCode::Right));
+        assert_eq!(app.settings_draft.error, "red");
     }
 
     #[test]