Where are my library, receipts and purchase history?

All under Account, in both the browser and the launcher:

• Library — everything you own, sortable, with install state shown when you are in the launcher.
• Transactions — every purchase with its date and what was paid, including ones that were refunded, which stay listed and are marked as such.
• Cart and Wishlist — what you are about to buy and what you are keeping an eye on.

Opening a purchase shows its receipt and, while it still qualifies, the option to request a refund.