Move organization admin settings to the sidebar disclosure

What does this MR do and why?

Behind the hide_unpinned_sidebar_items feature flag, this moves the Organization admin area's Settings section into the sidebar footer disclosure, where it renders as Organization settings, matching the Admin area.

It adds one entry (organization_admin → settings_menu) to the panel map introduced in !265260. The Organization settings label itself was already added in !270748 (merged) — this MR is what makes it reachable, by opting the panel into the disclosure.

The regular Organization sidebar (organization panel) is deliberately unchanged: it is excluded from PANELS_WITH_HIDEABLE_UNPINNED_ITEMS and is not in the panel map, so it keeps its in-place settings section and renders no disclosure.

Stack (4 of 6): footer placement → disclosure labels → Admin area → this MR → user settings → settings pins.

References

• Depends on: !265260
• Rollout issue: #604888
• Related: #621643

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. In the Rails console, enable the feature flag and the organization admin area:

   Feature.enable(:hide_unpinned_sidebar_items)
   Feature.enable(:org_stage_la_100)

2. Visit /o/<organization>/admin (for example /o/tanuki-inc/admin), or use the Manage organization button on the organization's pages.

3. Confirm an Organization settings button appears in the sidebar footer above Help, and Overview stays in the sidebar.

4. Open it and confirm it lists the organization admin settings pages.

5. Visit an organization admin settings page, for example /o/<organization>/admin/settings/general. Confirm the button shows as selected.

6. Collapse the sidebar to icon-only and confirm it becomes a gear with an Organization settings tooltip.

7. Visit the regular organization pages (for example /o/<organization>/groups_and_projects). Confirm they still show an in-place Settings section and no footer disclosure — the organization panel is intentionally not part of this change.

8. Disable the flag and confirm the organization admin area shows its in-place Settings section again.

MR acceptance checklist

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