improvement(docs): remove Ask AI, add the missing platform surfaces, align the sidebar (#6259)

* improvement(docs): remove Ask AI, add missing surfaces, align sidebar to the app

- Removes the Ask AI widget, its /api/chat route, and the four deps exclusive
  to it (@ai-sdk/openai, @ai-sdk/react, ai, streamdown). lib/embeddings and
  docsEmbeddings stay — /api/search uses them.
- Adds --surface-7 and --surface-hover, the last two platform surfaces docs
  lacked.
- Aligns the sidebar with the app's canonical nav chrome: px-2.5 -> px-2,
  text-small -> text-sm, hover --surface-3 -> --surface-active, and an active
  hover of --surface-6, matching the Chip the app's sidebar items are built on.

* fix(docs): make the sidebar hover CSS agree with the utilities

Review caught that the sidebar hover alignment in this PR had no visual
effect. `global.css` carries !important rules for both the link and button
sidebar items — they exist to beat fumadocs' own styles — and they were still
forcing the pre-alignment values: --surface-3 on an inactive hover, and
--surface-active on an active hover.

So the Tailwind utilities were dead on arrival. The global rules now carry the
app's values instead (--surface-active inactive, --surface-6 active), matching
the utilities rather than fighting them, with a comment noting the two must
move together.

apps/docs/components/docs-layout/sidebar-components.tsx

@@ -45,14 +45,16 @@ const ITEM_BASE =
 const ITEM_ACTIVE_MOBILE = 'bg-[var(--surface-active)] font-medium text-[var(--text-primary)]'
 
 const ITEM_DESKTOP =
-  'lg:mb-[0.0625rem] lg:block lg:rounded-lg lg:px-2.5 lg:py-1.5 lg:font-normal lg:text-small lg:leading-tight'
+  'lg:mb-[0.0625rem] lg:block lg:rounded-lg lg:px-2 lg:py-1.5 lg:font-normal lg:text-sm lg:leading-tight'
 const ITEM_TEXT = 'lg:text-[var(--text-body)]'
-const ITEM_HOVER = 'lg:hover:bg-[var(--surface-3)]'
-const ITEM_ACTIVE = 'lg:bg-[var(--surface-active)] lg:font-normal lg:text-[var(--text-body)]'
+const ITEM_HOVER = 'lg:hover:bg-[var(--surface-active)]'
+const ITEM_ACTIVE =
+  'lg:bg-[var(--surface-active)] lg:font-normal lg:text-[var(--text-body)] lg:hover:bg-[var(--surface-6)]'
 
 const FOLDER_TEXT = 'lg:text-[var(--text-body)] lg:font-medium'
-const FOLDER_HOVER = 'lg:hover:bg-[var(--surface-3)]'
-const FOLDER_ACTIVE = 'lg:bg-[var(--surface-active)] lg:text-[var(--text-body)]'
+const FOLDER_HOVER = 'lg:hover:bg-[var(--surface-active)]'
+const FOLDER_ACTIVE =
+  'lg:bg-[var(--surface-active)] lg:text-[var(--text-body)] lg:hover:bg-[var(--surface-6)]'
 
 export function SidebarItem({ item }: { item: Item }) {
   const pathname = usePathname()
@@ -133,7 +135,7 @@ export function SidebarFolder({ item, children }: { item: Folder; children: Reac
                 'flex flex-1 items-center gap-2 rounded-md px-2 py-1.5 text-sm transition-colors',
                 'text-[var(--text-muted)] hover:bg-[var(--surface-active)] hover:text-[var(--text-body)]',
                 active && ITEM_ACTIVE_MOBILE,
-                'lg:block lg:flex-1 lg:rounded-lg lg:px-2.5 lg:py-1.5 lg:text-small lg:leading-tight',
+                'lg:block lg:flex-1 lg:rounded-lg lg:px-2 lg:py-1.5 lg:text-sm lg:leading-tight',
                 FOLDER_TEXT,
                 !active && FOLDER_HOVER,
                 active && FOLDER_ACTIVE
@@ -146,7 +148,7 @@ export function SidebarFolder({ item, children }: { item: Folder; children: Reac
                 onClick={toggleOpen}
                 className={cn(
                   'rounded-md p-1 hover:bg-[var(--surface-active)]',
-                  'lg:cursor-pointer lg:rounded-md lg:p-1 lg:transition-colors lg:hover:bg-[var(--surface-3)]'
+                  'lg:cursor-pointer lg:rounded-md lg:p-1 lg:transition-colors lg:hover:bg-[var(--surface-active)]'
                 )}
                 aria-label={open ? 'Collapse' : 'Expand'}
               >
@@ -160,7 +162,7 @@ export function SidebarFolder({ item, children }: { item: Folder; children: Reac
             className={cn(
               'flex flex-1 items-center gap-2 rounded-md px-2 py-1.5 text-sm transition-colors',
               'text-[var(--text-muted)] hover:bg-[var(--surface-active)]',
-              'lg:flex lg:w-full lg:cursor-pointer lg:items-center lg:justify-between lg:rounded-lg lg:px-2.5 lg:py-1.5 lg:text-left lg:text-small lg:leading-tight',
+              'lg:flex lg:w-full lg:cursor-pointer lg:items-center lg:justify-between lg:rounded-lg lg:px-2 lg:py-1.5 lg:text-left lg:text-sm lg:leading-tight',
               FOLDER_TEXT,
               FOLDER_HOVER
             )}