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/types/auth.ts

@@ -10,6 +10,7 @@ export interface User {
   force_password_reset: boolean
   is_active: boolean
   allowed_library_ids?: string[]
+  pinned_library_ids?: string[]
   is_default_admin?: boolean
   is_protected?: boolean
   realtime_online?: boolean