Add category filters to the feature library modal

What does this MR do and why?

Adds a row of category filter buttons to the feature library modal so users can quickly narrow the catalog to a single category (for example Plan, Code, Manage).

• Selecting a category scopes the results to that category and drops the collapsible section headers in favour of a flat, filtered grid.
• Selecting the active category again (or All) restores the grouped, collapsible view.
• The active category is conveyed by the button's selected state.
• On narrow viewports the filter row scrolls horizontally instead of wrapping, using utility classes only (no custom CSS).

It also adds a new click_category_filter_in_feature_library_modal internal event that fires when a category is selected (labelled with the category id). A category tracking event previously existed (click_category_tab_in_feature_library_modal) but was removed when the category tabs were replaced with collapsible sections in !246751 (merged). Rather than resurrecting the removed definition, this MR introduces a distinct event so the instrumentation metadata (milestone / introduced_by_url) stays accurate for the new filter UI.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Open the super sidebar and launch the Feature Library modal.
2. Confirm the category filter buttons render below the search box.
3. Select a category and confirm the grid filters to it and the section headers disappear.
4. Select the same category again (or All) and confirm the grouped view returns.
5. On a narrow window, confirm the filter row scrolls horizontally.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist.