Scrollable dropdown doesn't highlight current selection & keyboard navigation is broken #127

Description of the bug

The scrollable dropdown component (DsDynamicScrollableDropdownComponent) has these usability issues:

1. No visual indication of current selection: When opening the dropdown, the currently selected option is not highlighted/marked as active
2. Broken keyboard navigation: Arrow keys don't properly navigate through all available options (including the "Clear selection" button)

To Reproduce

Quick Test Steps:

1. Navigate to any form with a scrollable dropdown field
2. Select an option from the dropdown and save
3. Return to edit the form and open the same dropdown
4. Expected: The currently selected option should be visually highlighted
5. Actual: incorrect option appears selected/highlighted
6. Try using arrow keys to navigate and Enter to select
7. Expected: Arrow keys should navigate through all options including "Clear selection"
8. Actual: Navigation skips the clear option and indexing is incorrect

Detailed Test Case: Dropdown Selection

Setup:

1. Find any form with a scrollable dropdown
2. Ensure the dropdown has multiple options

Test Steps:

1. Select Initial Value: Choose any option from the dropdown (e.g., "Option A")
2. Verify Current Selection: Open the same dropdown again
3. Expected: "Option A" should be visually highlighted as the currently selected option
4. Actual: Wrong option appears highlighted
5. Change Selection: Select a different option (e.g., "Option B")
6. Verify New Selection: Open the dropdown again
7. Expected: "Option B" should now be highlighted
8. Test Keyboard Navigation: Use arrow keys (↑/↓) to navigate through options
9. Expected: Should navigate through all options including "Clear selection" button
10. Actual: Navigation skips "Clear selection" options

Expected Results:

• The currently selected option is always correctly highlighted in the dropdown with .active class
• After changing selections, the new option is properly highlighted
• Keyboard navigation works smoothly through all available options
• This behavior is consistent across all scrollable dropdowns

Technical Details

Component affected: DsDynamicScrollableDropdownComponent
Files involved:

• dynamic-scrollable-dropdown.component.html
• dynamic-scrollable-dropdown.component.ts

Root cause:

• Missing logic to set selectedIndex to match current value when dropdown opens
• Incorrect index calculation when "Clear selection" button is present (options need +1 offset)
• scrollToSelected() includes disabled items in selection

Proposed solution:

• Add setSelectedIndexToCurrentValue() method to highlight current selection on dropdown open
• Treat "Clear selection" as index 0, shift regular options to index+1
• Update keyboard navigation to handle the full range including clear button
• Fix Enter key logic to handle clear selection (index 0) vs regular options