Publish a stable stylesheet URL, move bookshop links to /shop

Publishes a second, non-fingerprinted copy of main.css at /css/main.min.css
alongside the fingerprinted one the site itself links — same source
resource, so bookshop's ARC/download pages (now mounted at
thistleandmoth.org/shop, not shop.thistleandmoth.org) can link real,
always-current CSS without tracking a hash that changes on every deploy.
Updates the two remaining shop.thistleandmoth.org references to the new
canonical URL.

hugo.toml

@@ -13,7 +13,7 @@ disableKinds = ["taxonomy", "term"]
   description = "TODO: one-line description of Thistle & Moth"
   author = "Thistle & Moth"
   eec_base_url = "https://eec.thistleandmoth.org"
-  bookshop_base_url = "https://shop.thistleandmoth.org"
+  bookshop_base_url = "https://thistleandmoth.org/shop"
 
 [[menu.main]]
   name = "Submissions"