[JEWEL-1007] Improve ListComboBox empty state

[faogustavo]

• Updated the list combo box to match swing implementation
  • If the list is empty, it should have the same height from the combobox

Evidences

Before	After

Jewel	UI Sandbox

Release notes

⚠️ Important Changes

• Updated ComboBox padding values to match Swing
  • Getting from Popup.Body insets
  • Defaults to 8.dp instead of 6.dp

Bug fixes

• ListComboBox now matches Swing when the list is empty

---

Note

Aligns ComboBox popup padding with Swing and makes empty ListComboBox popups at least as tall as the combobox.

• ComboBox styling:
  • popupContentPadding aligned with Swing: retrieve Popup.Body top/bottom insets in IntUiBridgeComboBox, defaulting to 8.dp.
  • Standalone defaults (ComboBoxMetrics.default/undecorated) popupContentPadding increased from 6.dp to 8.dp.
• ListComboBox behavior:
  • Track full combobox DpSize (width + height) and use width as fallback maxPopupWidth.
  • Popup list now applies padding only when items exist; for empty lists, enforces minimum height = combobox height + vertical padding.
• Samples/Tests:
  • Showcase adds an "Empty Combo-box" example.
  • UI tests updated/added to verify popup height equals maxPopupHeight and that empty popups are at least combobox height + padding.

^{Written by Cursor Bugbot for commit 603bb69. This will update automatically on new commits. Configure here.}

[rock3r]

Ready to merge