add follows autocomplete source

src/features/autocomplete/useFollowsSource.ts

@@ -0,0 +1,49 @@
+import {useEffect} from 'react'
+
+import {useProfileFollowsQuery} from '#/state/queries/profile-follows'
+import {useSession} from '#/state/session'
+import {type LocalSource} from '#/components/Autocomplete/types'
+
+/*
+ * The profile-follows query key ignores `limit`, so this must use the default
+ * page size or it would collide with existing default-limit callers (DM
+ * flows, SearchablePeopleList). Two default pages ≈ 60 profiles.
+ */
+const MAX_PAGES = 2
+
+/**
+ * The current account's follows as an autocomplete LocalSource, for instant
+ * local mention matches while the remote typeahead request is in flight.
+ * Empty when logged out.
+ */
+export function useFollowsSource(): LocalSource {
+  const {currentAccount} = useSession()
+  const {data, hasNextPage, isFetching, isError, fetchNextPage} =
+    useProfileFollowsQuery(currentAccount?.did)
+
+  const pages = data?.pages ?? []
+
+  useEffect(() => {
+    if (
+      pages.length > 0 &&
+      pages.length < MAX_PAGES &&
+      hasNextPage &&
+      !isFetching &&
+      !isError
+    ) {
+      void fetchNextPage()
+    }
+  }, [pages.length, hasNextPage, isFetching, isError, fetchNextPage])
+
+  return {
+    key: 'follows',
+    items: pages.slice(0, MAX_PAGES).flatMap(page =>
+      page.follows.map(profile => ({
+        key: profile.did,
+        type: 'profile' as const,
+        value: '@' + profile.handle,
+        profile,
+      })),
+    ),
+  }
+}