feat: give each Site product and the cart its own page

The home, each product's Montagem and the cart now have their own
addresses (/artes-avulsas, /arquivo-por-metro, /uv-artes-avulsas,
/uv-arquivo-por-metro, /carrinho) and show only their own content, with
Back, Forward, reload and direct links working as in any store. They stay
one document so uploaded artworks survive moving between pages; nginx
serves index.html for these addresses.

"Adicionar ao carrinho" puts the item in the cart and opens it, and an
empty cart says so. Portal quote links open in the cart. Also fixes the
"57 cm" line break on the ready-sheet option, returns "Novo pedido" to
the home, and says PDF depends on the product.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

deploy/nginx.conf.template

@@ -48,6 +48,11 @@ server {
     }
     # Always revalidate HTML/JS/CSS after a deployment. Without this, a browser
     # can pair a new Kanban page with a cached older script after a rollout.
+    # The Site's product pages and cart are addresses of the same page (web/site-pages.js).
+    location ~ ^/(arquivo-por-metro|artes-avulsas|uv-arquivo-por-metro|uv-artes-avulsas|carrinho)/?$ {
+        expires -1;
+        try_files /index.html =404;
+    }
     location / {
         expires -1;
         try_files $uri $uri/ =404;