Refine feature library modal layout and copy

What does this MR do and why?

Refines the Feature Library modal (behind the feature_library_modal feature flag) with a set of layout and copy improvements:

• Replaces the category tabs with collapsible sections, one per category, each with a chevron toggle (expanded by default).
• Reworks the feature cards: subtle border, icon stacked above the title, full-width title/description, and a footer holding the subscription tier (plain text, defaulting to "Free") on the left and the pin button on the right.
• Adds a scroll scrim (reusing the shared ScrollScrim component) so top/bottom fade indicators appear when content overflows, and adjusts modal padding so the scrim reaches the edges.
• Drops the background blur behind the modal.
• Makes copy context-aware: the search placeholder now reads "Search features in this project" / "...group" based on the panel, and the feedback link references the feature library.

References

More Features Modal User feedback (#604008) • Austin Regnery

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable the feature flag in the Rails console:

   Feature.enable(:feature_library_modal)

2. Visit any project or group and open the super sidebar.
3. Open the Feature Library modal (keyboard shortcut, or the entry point in the sidebar).
4. Confirm the collapsible sections, card footer with tier text, the scroll scrim on overflow, the context-aware search placeholder, and the updated feedback link copy.

MR acceptance checklist

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