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.

hugo.toml

@@ -13,6 +13,10 @@ disableKinds = ["taxonomy", "term"]
   author = "Will Estes"
   description = "Two Squirrels Press — character-driven speculative fiction and poetry."
   eec_base_url = "https://eec.twosquirrelspress.com"
+  bookshop_base_url = "https://shop.twosquirrelspress.com"
+  # Publishable key is safe to commit (it's meant to be public) — fill in
+  # the real key once the Stripe account is set up.
+  stripe_publishable_key = ""
 
 [[menu.main]]
   name = "Books"