Add Stripe embedded checkout to book pages

Replaces the gumroad_url stub with an on-site embedded checkout (buy
button -> POST to the bookshop backend -> mounted Stripe iframe),
following the existing contact/submissions form's fetch/status-message
pattern. Adds thank-you and resend-link pages, an example book template
(draft), and updates the privacy policy and terms to honestly disclose
Stripe as a third-party payment processor.

content/terms.md

@@ -5,14 +5,26 @@ description: "Terms for using twosquirrelspress.com."
 
 _Last updated: July 2026_
 
-These terms cover using twosquirrelspress.com and submitting the contact
-form.
+These terms cover using twosquirrelspress.com, submitting the contact or
+submissions form, and buying a book.
 
 ## Using this site
 
 This site is provided as-is, for informational purposes about Two Squirrels
 Press, its books, and its authors.
 
+## Buying a book
+
+Payments are processed by Stripe, acting as merchant of record — Stripe
+handles your card details, sales tax, and VAT directly. Two Squirrels
+Press never sees or stores your card number.
+
+Ebooks you buy are for your own personal use — please don't redistribute
+or resell the files. Because these are digital downloads, all sales are
+final except where required by law; if something's gone wrong with your
+purchase (wrong file, failed download, etc.) [get in touch](/contact/)
+and we'll sort it out.
+
 ## No warranty
 
 This site and its content are provided without warranties of any kind,