docs(ui): correct the navigation item type count and document separator

The app shell doc claimed eight navigation item types and enumerated eight,
omitting `separator` — the schema has declared nine since the inverse-drift
fix (#1878/#1891/#1894) that added it to match the objectui renderer's
`item.type === 'separator'` branch.

The gap matters more now that this PR discriminates the union on `type`: a
mistyped `type` reports the valid discriminator list, so the doc's enumeration
is the thing authors check it against. And `SeparatorNavItemSchema` is
`.strict()` over only `type`/`id`/`order` — it does not spread
`BaseNavItemSchema` — so `label`/`icon`/`badge`/`visible`/`requiredPermissions`
on a separator are now hard errors where they used to be silently stripped.
The "all navigation items share these base properties" table was the exact
sentence that would send an author into that error, so it now names the
exception.

Both claims asserted against the live schema before writing: the union has
nine members, and a separator accepts type/id/order while rejecting each of
the five base props.

Refs #4001

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0147tNF4Snk7Ry1KGt4a5PY4

Author: claude

content/docs/ui/apps.mdx

@@ -57,7 +57,7 @@ const crmApp = {
 
 ## Navigation Items
 
-The navigation tree supports eight item types, combined to create rich menu structures. The most common are shown below (`object`, `dashboard`, `page`, `url`, `group`); the spec also defines `report`, `action`, and `component` items.
+The navigation tree supports nine item types, combined to create rich menu structures. The most common are shown below (`object`, `dashboard`, `page`, `url`, `group`, `separator`); the spec also defines `report`, `action`, and `component` items.
 
 ### Object Navigation
 
@@ -121,9 +121,23 @@ Groups items into collapsible sections with children:
 }
 ```
 
+### Separator
+
+A visual divider in the navigation list. It renders no target and carries no
+label — the only keys it accepts are `type`, an optional `id`, and an optional
+`order`:
+
+```typescript
+{ type: 'separator', order: 30 }
+```
+
 ### Common Navigation Properties
 
-All navigation items share these base properties. Every item **must** declare a unique `id` (lowercase `snake_case`) — it is required by the schema and is referenced by `homePageId` and `mobileNavigation.bottomNavItems`:
+All navigation items **except `separator`** share these base properties. Every
+item **must** declare a unique `id` (lowercase `snake_case`) — it is required by
+the schema and is referenced by `homePageId` and `mobileNavigation.bottomNavItems`.
+(On a `separator`, `id` is optional and the remaining properties below are
+rejected — a divider has nothing to label, gate, or badge.)
 
 | Property | Type | Description |
 | :--- | :--- | :--- |