[FEATURE] Add Dropdown Support to BitToolbar Component #99

Add support for dropdown items inside the BitToolbar component, following the Bootstrap Italia specification for Toolbar con Dropdown.

This feature allows toolbar items to render as dropdown buttons with a contextual submenu instead of plain links. It should work across all three toolbar sizes (large, medium, small) and support the full set of item states (active, disabled, badge).

Requirements:

• Add a BitToolbarDropdownItem component (or extend BitToolbarItem) that renders a <div class="dropdown"> wrapping a <button class="btn btn-dropdown dropdown-toggle"> as the trigger
• The trigger button must carry the required Bootstrap Italia attributes: data-bs-toggle="dropdown", aria-haspopup="true", aria-expanded="false", and a title parameter
• Render the dropdown menu as a <div class="dropdown-menu"> containing a <div class="link-list-wrapper"><ul class="link-list"> with <a class="dropdown-item list-item"> entries
• Support all three toolbar size variants (default / .toolbar-medium / .toolbar-small) — icon and label visibility follows the same rules as regular toolbar items
• Support the active state on the trigger button (adds .active class)
• Support the disabled state on the trigger button (adds .disabled + aria-disabled="true")
• Support optional badge on the dropdown trigger (same badge-wrapper + toolbar-badge pattern as regular items)
• Support the "more actions" variant via a toolbar-more class on the trigger button (uses the it-more-actions icon)
• Expose the dropdown items list as a RenderFragment or child-content parameter
• Ensure correct ARIA: aria-labelledby on the dropdown menu pointing to the button id

Acceptance Criteria:

• Add tests
• Add documentation
• Dropdown toolbar items render correctly for all three size variants (large, medium, small)
• Active and disabled states are correctly reflected in both CSS classes and ARIA attributes
• Badge integration works on dropdown triggers (numeric for large, dot for medium/small)
• Keyboard navigation follows Bootstrap Italia / WAI-ARIA menu button pattern (Enter/Space to open, Arrow keys to navigate items, Escape to close)
• Focus is returned to the trigger button after closing the dropdown via keyboard
• A story file is added for each size variant and state combination

References

• https://italia.github.io/bootstrap-italia/docs/menu-di-navigazione/toolbar/#toolbar-con-dropdown
• https://www.w3.org/WAI/ARIA/apg/patterns/menu-button/