Replace sidebar display options with Compact and Full

What does this MR do and why?

The sidebar now has two layout options instead of three. This is behind the hide_unpinned_sidebar_items feature flag.

Before, users chose between Flat, Organized, and Everything. That asked them to pick a layout before they had enough pins for the choice to matter.

Option	What it shows
Compact (default)	Pinned items only. A category with 3 or more pins is grouped automatically. Pins from smaller categories stay in a flat list above the groups, in the order the user pinned them.
Full	Same as the old "Everything": a flat list of pins, then all categories.

• Full keeps the stored value all_categories, so users who picked "Everything" land in Full.
• The retired values pinned_only and grouped_pins fall back to Compact.
• The tracking event now reports compact or all_categories as the selected option.

Known limitation (not new): if you open a pinned page through a regular link and its group is collapsed, nothing in the sidebar marks it as the current page. The old Organized option had the same problem. We'll handle it in a follow-up.

References

• Feature issue: #601398 (closed)
• Feature flag rollout: #604888
• MR that added the tracking event: !253007 (merged)

Screenshots or screen recordings

How to set up and validate locally

1. Enable the feature flag in the Rails console: Feature.enable(:hide_unpinned_sidebar_items)
2. Go to any project. Open the Customize sidebar menu (the ... button next to the project name) and check that it has two options: Compact and Full.
3. In Compact, pin 2 items from one category (for example Code). They show in the flat pinned list.
4. Pin a third item from the same category. All 3 move into a Code group below the flat list.
5. Unpin one of them. The remaining 2 go back to the flat list.
6. Switch to Full. All pins show in one flat list, followed by all categories.
7. Collapse the sidebar to icons only. In Compact, a divider separates the flat pins from the grouped ones.

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