Fix payment page mobile layout — remove conflicting flex container

The cart.css had `display:flex; align-items:center` on main:has(folk-payment-page)
which fought with the component's own `width:100%; margin:0 auto` centering:
- flex align-items:center shrunk the subnav to content width
- padding:0 0.5rem overrode shell.css padding-top:92px (desktop)
- Redundant since all components already self-center with margin:auto

Also hide the shop subnav on public-facing pages (/pay/:id, /group-buy/:id)
since payers/pledgers don't need Carts/Catalog/Orders navigation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

modules/rcart/components/cart.css

@@ -4,15 +4,16 @@ main {
 	padding: 0;
 }
 
-/* Center narrow form pages (payment request, payment page, group buy, dashboard) */
-main:has(folk-payment-request),
-main:has(folk-payment-page),
-main:has(folk-group-buy-page),
-main:has(folk-payments-dashboard) {
-	display: flex;
-	flex-direction: column;
-	align-items: center;
-	padding: 0 0.5rem;
+/*
+ * Narrow page components (payment page, request form, dashboard, group buy)
+ * already set their own max-width + margin:0 auto for centering.
+ * We just need to ensure the subnav still spans full width above them.
+ */
+
+/* Hide the module subnav on public-facing pages — payers/pledgers don't need shop nav */
+main:has(folk-payment-page) .rapp-subnav,
+main:has(folk-group-buy-page) .rapp-subnav {
+	display: none;
 }
 
 @media (max-width: 600px) {