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>

PROGRESS.md

@@ -59,16 +59,16 @@ Issues found during hands-on testing of the deployed prod site on mobile and des
 - [x] Footer social icons should match the "Find me on" icons from the contact page
 
 ### Collections / all products
-- [ ] Categories on all-products page are too spaced out
+- [x] Categories on all-products page are too spaced out
 
 ### Content pages
-- [ ] Hero title spacing differs between content pages (about, delivery, etc.) — contact is fine
+- [x] Hero title spacing differs between content pages (about, delivery, etc.) — contact is fine
 
 ### Sale / filtering
-- [ ] Should there be a "Sale" section or filter for discounted products?
+- [x] Should there be a "Sale" section or filter for discounted products?
 
 ### Errors
-- [ ] 404 page is broken
+- [x] 404 page is broken (CSS path was wrong — `/assets/app.css` instead of `/assets/css/app.css`)
 
 ### Search (deferred — after usability fixes)
 - [ ] Search doesn't work (modal opens but no results/functionality) — see [docs/plans/search.md](docs/plans/search.md)