Wire up real Stripe sandbox test key and example book price ID

From the Two Squirrels Press Stripe sandbox account created and walked
through Managed Payments setup for. Both values are test-mode / safe
to commit; swap to live-mode equivalents at go-live.

hugo.toml

@@ -14,9 +14,10 @@ disableKinds = ["taxonomy", "term"]
   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 = ""
+  # Publishable key is safe to commit (it's meant to be public). This is
+  # the Two Squirrels Press sandbox/test-mode key — swap to the live-mode
+  # publishable key as part of go-live (see the plan's verification steps).
+  stripe_publishable_key = "pk_test_51TtYRgQEDYBEXYYF870hcDq58pl5MgT9evlYdgVE19VVXzNGA1DnVUX0lDTFpKJGShUHDT6NlIhiqz7u6YHPvX6x00rKIk2T3v"
 
 [[menu.main]]
   name = "Books"