fix collection filters, hero spacing, sale filter, and error page CSS

- collection filter bar: horizontal scroll on mobile instead of wrapping
  across 3 rows, smaller pills at mobile sizes
- add sale collection filter at /collections/sale (filters on_sale products)
- hero :page variant: add consistent top padding (var(--space-2xl))
- contact page: remove redundant top padding (hero handles it now)
- error page: fix CSS path from /assets/app.css to /assets/css/app.css
  (broken in production due to asset fingerprinting)

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

lib/simpleshop_theme_web/controllers/error_html.ex

@@ -66,7 +66,7 @@ defmodule SimpleshopThemeWeb.ErrorHTML do
         <meta charset="utf-8" />
         <meta name="viewport" content="width=device-width, initial-scale=1" />
         <title>{@error_code} - {@error_title}</title>
-        <link phx-track-static rel="stylesheet" href={~p"/assets/app.css"} />
+        <link phx-track-static rel="stylesheet" href={~p"/assets/css/app.css"} />
         <style id="theme-css">
           <%= Phoenix.HTML.raw(@generated_css) %>
         </style>