Add more info to NC Mall section of Item Getting Guide

NC prices, some CSS, and also a new application-level helper that adds
a feature I've long wanted and been working around for Turbo: the
ability to specific that a stylesheet is specific to the current page,
and should be unloaded when removed!

I use this to write `sources.sass` without the usual
`body.items-sources` scoping that we've historically used to control
what pages a stylesheet applies to. (In the long past, this was because
a lot of stylesheets were—and still are–routed through the
`application.sass` stylesheet! But even for more recent standalone page
stylesheets, I've done the scoping, to avoid issues with styles leaking
beyond the page they're meant for when Turbo does a navigation.)

app/assets/stylesheets/items/sources.sass

@@ -0,0 +1,14 @@
+.nc-mall-items
+	list-style: none
+	display: flex
+	flex-wrap: wrap
+	gap: 1em
+
+	li
+		display: flex
+		flex-direction: column
+		align-items: center
+		width: auto
+
+	.add-to-cart
+		font-size: 85%

app/helpers/application_helper.rb

@@ -175,6 +175,14 @@ module ApplicationHelper
     end
   end
 
+  # Includes a stylesheet designed for this specific page of the app, which
+  # should be removed when navigating to another page. We use Turbo's
+  # `data-turbo-track="dynamic"` option to do this.
+  def page_stylesheet_link_tag(src, options={})
+    options = {data: {"turbo-track": "dynamic"}}.deep_merge(options)
+    stylesheet_link_tag src, options
+  end
+
   def secondary_nav(&block)
     content_for :before_flashes,
       content_tag(:nav, :id => 'secondary-nav', &block)

app/models/item.rb

@@ -175,6 +175,10 @@ class Item < ApplicationRecord
     @wanted || false
   end
 
+  def current_nc_price
+    nc_mall_record.current_price
+  end
+
   def restricted_zones(options={})
     options[:scope] ||= Zone.all
     options[:scope].find(restricted_zone_ids)

app/models/nc_mall_record.rb

@@ -1,3 +1,7 @@
 class NCMallRecord < ApplicationRecord
   belongs_to :item
+
+  def current_price
+    discount_price || price
+  end
 end

app/views/items/sources.html.haml

@@ -3,7 +3,16 @@
 
 - if @nc_mall_items.present?
 	%h2 NC Mall items
-	= render @nc_mall_items
+	%p
+		These items are available in the NC Mall right now! You can buy them
+		with #{link_to "Neocash", "https://secure.nc.neopets.com/get-neocash"},
+		a special currency you can purchase directly from Neopets or redeem via gift cards.
+	%ul.nc-mall-items
+		- @nc_mall_items.each do |item|
+			%li
+				= render item
+				%button.add-to-cart{onclick: "alert('Todo!')"}
+					Buy (#{item.current_nc_price} NC)
 
 - if @np_items.present?
 	%h2 Neopoint items
@@ -16,3 +25,6 @@
 - if @other_nc_items.present?
 	%h2 Other NC items
 	= render @other_nc_items
+
+- content_for :stylesheets do
+	= page_stylesheet_link_tag "items/sources"