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

[ahliweb]

What does this PR do?

Implements a comprehensive admin sidebar redesign that adds collection grouping, plugin subgroups, nested submenus, icon rendering, sidebar configuration, and bidirectional sync with public menus.

Discussion: #1027

Data Layer

• Migration 039: Adds sort_order (INTEGER, default 0) and group (TEXT, nullable) columns to _emdash_collections
• Collection types: sortOrder, group, and icon added to Collection, CreateCollectionInput, UpdateCollectionInput, ManifestCollection
• Plugin types: group and sortOrder added to PluginAdminPage and ManifestPlugin.adminPages
• Ordering: Collections now ordered by sort_order ASC, slug ASC instead of just slug ASC

Admin Sidebar Rendering

• Collection grouping: Collections with a group field render in separate collapsible sidebar groups; ungrouped collections remain in the default "Content" group
• Plugin grouping: Plugin pages with a group field render in separate collapsible sidebar groups; ungrouped pages remain in "Plugins"
• Icon resolution: resolveIcon() helper maps 30+ icon names to Phosphor components with fallback to defaults
• Nested submenus: NavItem.children support with NavSubMenu expandable component (max 1 level of nesting)
• Sidebar config: hideCoreFeatures and hideCollections options in EmDashConfig.sidebar to hide items from the nav
• RTL-safe: All layout uses logical Tailwind classes (ms-*, ps-*, start-*); chevrons flip in RTL

Menu Sync

• addToMenu: New field on CreateCollectionInput — auto-adds a type: "collection" menu item to the specified public menu on collection creation
• Auto-cleanup: Deleting a collection automatically removes its menu items
• Manual sync endpoint: POST /_emdash/api/schema/collections/:slug/sync-menu to add a collection to a menu after creation
• Full sync engine:
  • GET /_emdash/api/menus/:name/sync-diff — preview changes (toAdd, toRemove, toReorder)
  • POST /_emdash/api/menus/:name/sync — apply changes to align menu with sidebar structure

Reordering UI

• CollectionReorderDialog: Drag-and-drop dialog using @dnd-kit/core + @dnd-kit/sortable for reordering collections
• API endpoint: POST /_emdash/api/schema/collections/reorder — batch updates sort_order for multiple collections
• Integration: "Reorder" button added to Content Types admin page

Related PR

• Implementation: feat(admin): sidebar menu tree with collection grouping, plugin subgroups, and public menu sync #1024

Type of change

• Feature (requires maintainer-approved Discussion)

[ahliweb]

@ascorbic Could you please add the appropriate labels to this issue? Suggested labels: area/admin, area/core, area/plugins, enhancement. Thanks!