Contextual collection workflows in the admin sidebar (small first slice of #1763)

[logelog]

Problem

The current sidebar keeps a collection and the workflows that belong to it in separate flat sections. A common example is a posts collection: editors see Posts under Content, while its assigned Categories and Tags appear later under Manage. ?All posts? and ?Add new? are also page-level actions rather than a discoverable collection workflow.

This makes the navigation harder to scan as a site grows, and it is especially awkward for editors coming from mature publishing tools where collection-specific actions live together.

This proposal is deliberately a small, UI-only first slice of #1763 and the closed #1024. It does not add collection groups, ordering columns, public-menu sync, or new database/config schema.

Proposed v1

For a collection that has assigned taxonomies, render one Kumo-native collapsible submenu below the collection item:

• All {collection}
• Add new
• only the taxonomies assigned to that collection

Behavior:

• In the expanded sidebar, use the collection row as the disclosure trigger and expose the collection destination as All {collection}; in the collapsed sidebar, link the parent directly to the collection.
• Expand the submenu when the parent or one of its children is the active route.
• Preserve exact route matching so sibling collections do not become active accidentally.
• Apply the existing role checks to every nested item.
• Keep a collapsed sidebar usable by linking the parent directly.
• Use Kumo's existing Collapsible and SidebarMenuSub primitives, including their connector/active-state treatment, instead of introducing custom sidebar chrome.
• Preserve keyboard navigation, focus visibility, RTL-safe spacing, and reduced-motion behavior.

Collections without assigned taxonomies keep the current single-link rendering, so this does not add empty navigation.

Why this scope

#1024 demonstrated that grouping, ordering, menu sync, nested navigation, and schema changes in one PR are too broad to review safely. This slice can improve the everyday editorial path without a migration or new public API, and it leaves the larger #1763 data model undecided.

Prototype

A focused implementation is available on my fork for design/API review:

• Branch: https://github.com/logelog/emdash/tree/feat/admin-sidebar-navigation-ux
• Current diff is limited to the admin sidebar model/rendering, tests, and an admin changeset.
• Validation: admin typecheck passed; 23 targeted browser tests passed; git diff --check passed.
• The branch contains no site-specific branding or private plugin code.

If maintainers agree with this narrow direction, I will open a draft PR linked to this Discussion and keep any broader grouping/reordering work separate.

Open questions

1. Is ?only show children for collections with assigned taxonomies? the right v1 trigger?
2. Should Add new remain in the submenu, or should v1 include only All plus assigned taxonomies?
3. Is the Kumo connector treatment desirable here, or should child items be indented without a visible guide?