Admin sidebar menu tree with collection grouping, plugin subgroups, and public menu sync

[ahliweb]

Problem

The admin sidebar currently renders all collections as a flat alphabetical list under a single Content group. There is no way to:

1. Group collections (e.g., Blog, Shop, Team) into separate collapsible sidebar sections
2. Control collection order — everything is sorted alphabetically by slug with no user-configurable ordering
3. Organize plugin admin pages — all plugin pages are dumped into a single Plugins group with no grouping or custom icons
4. Create nested submenus — no way to have hierarchical navigation within sidebar groups
5. Hide unused core features — sites that do not use comments, redirects, widgets, etc. still see them in the sidebar
6. Sync sidebar structure to public menus — the admin sidebar and public navigation are completely separate systems

Proposed Solution

Data Layer

• Add sort_order and group columns to _emdash_collections
• Extend PluginAdminPage with group and sortOrder fields
• Pass icon, group, sortOrder through to the manifest

Admin Sidebar

• Group collections by group field into collapsible sections
• Plugin pages grouped similarly
• Icon resolution (30+ Phosphor icons)
• One level of nested submenus
• hideCoreFeatures / hideCollections config option

Menu Sync

• addToMenu on collection creation
• Auto-cleanup on collection deletion
• Full sync engine: diff + apply endpoints

Reordering UI

• Drag-and-drop collection reordering dialog
• POST /_emdash/api/schema/collections/reorder endpoint

Implementation

A PR has been prepared with all changes: #1024

The implementation includes 28 new tests and follows all existing patterns (Kumo components, Lingui i18n, RTL-safe classes, API envelope consistency).

[ahliweb]

@ascorbic All CI checks are now green on the related PR (#1024). Ready for review when you have a moment.

Summary:

• Admin sidebar with collection grouping, plugin subgroups, nested submenus, and icon resolution
• Menu sync engine - bidirectional sync between sidebar structure and public menus
• Drag-and-drop reordering UI for collections with batch API endpoint
• 28 new tests across collection ordering and menu sync

[ahliweb]

This is important for grouping plugin submenus so we can use it as a foundation for developing future plugins.
I am waiting for this approval so I can develop several plugins that need this feature soon.

[ahliweb]

Implementation is now live in PR #1024. All CI checks pass (Tests, Typecheck, Lint, E2E, Smoke, Integration, Browser). Ready for maintainer review.

[ascorbic]

Hi @ahliweb. Support for collection grouping is an interesting idea, but I think this is more than one feature, and shouldn't be done as one block. I'm particularly unsure about the menu sync feature. Can you explain a bit more about what it's doing and what it's for? Is this a required part of the feature?

The reason we ask for an approved discussion before opening a PR is so we can discuss these details before you spend the time on it.