Implement real My Shelf logic

[ilsea]

• Make an anonymous ID, store it in localStorage, and send it as the x-user-id header in client.ts.
• Read the shelf from the server: myEntry in the queries, myShelf for the shelf page.
• Use the mutations setShelved, setRating, setNote instead of the mock useState in BrowsePage and BookDetailPage.
• BookGrid still imports the ShelfEntry type from mockBooks.