feat: persist pinned libraries per user with cross-device sync (#20)

Store pinned library IDs on the user record and expose GET/PUT
/me/pinned-libraries endpoints. The web client now loads and saves pins
through the API, migrates legacy localStorage data once, and applies
pinned ordering on both the libraries and home pages.

Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: truewhile <truewhile@users.noreply.github.com>

web/src/hooks/usePinnedLibraries.ts

@@ -0,0 +1,52 @@
+import { useCallback, useEffect, useState } from 'react'
+
+import {
+  loadPinnedLibraryIds,
+  savePinnedLibraryIds,
+  togglePinnedLibraryId,
+} from '../utils/pinnedLibraries'
+
+export function usePinnedLibraries() {
+  const [pinnedIds, setPinnedIds] = useState<string[]>([])
+  const [loading, setLoading] = useState(true)
+  const [syncing, setSyncing] = useState(false)
+
+  useEffect(() => {
+    let cancelled = false
+    setLoading(true)
+    loadPinnedLibraryIds()
+      .then((ids) => {
+        if (!cancelled) setPinnedIds(ids)
+      })
+      .catch(() => {
+        if (!cancelled) setPinnedIds([])
+      })
+      .finally(() => {
+        if (!cancelled) setLoading(false)
+      })
+    return () => {
+      cancelled = true
+    }
+  }, [])
+
+  const togglePin = useCallback(async (libraryId: string) => {
+    let previous: string[] = []
+    let optimistic: string[] = []
+    setPinnedIds((current) => {
+      previous = current
+      optimistic = togglePinnedLibraryId(current, libraryId)
+      return optimistic
+    })
+    setSyncing(true)
+    try {
+      const saved = await savePinnedLibraryIds(optimistic)
+      setPinnedIds(saved)
+    } catch {
+      setPinnedIds(previous)
+    } finally {
+      setSyncing(false)
+    }
+  }, [])
+
+  return { pinnedIds, loading, syncing, togglePin }
+}