replace Tailwind in content + collection, remove shop Tailwind entirely (Phase 5c)

- Replace all Tailwind utilities in content.ex and collection.ex with
  semantic CSS classes (content body, contact form, cards, reviews, etc.)
- Delete app-shop.css (Tailwind shop entry point)
- Remove shop Tailwind config from config.exs, dev.exs, mix.exs
- Remove shop Tailwind stylesheet link from shop_root.html.heex
- Add collection filter bar, empty state, and select dropdown styles
- Fix filter pill sizing (use theme font vars instead of hardcoded rem)
- Fix active pill contrast (tinted accent background + dark accent text)
- Fix --t-text-on-accent fallback for pill legibility
- Add padding/font-size to .themed-select

Shop pages now use zero Tailwind. Admin Tailwind remains for Phase 6.

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

assets/css/shop.css

@@ -1,16 +1,16 @@
 /* Shop CSS — hand-written, zero-framework stylesheet.
-   Layered cascade: later layers beat earlier ones, no !important needed.
-
-   This file runs alongside app-shop.css during migration (Phases 1-4).
-   After Phase 5 it replaces app-shop.css entirely. */
+   Layered cascade: later layers beat earlier ones, no !important needed. */
 
 @layer reset, primitives, tokens, components, layout, utilities, overrides;
 
 @import "./shop/reset.css";
 
-/* Theme CSS stays in the existing files (loaded via app-shop.css).
-   Primitives and tokens will be wrapped in @layer when
-   app-shop.css is removed in Phase 5. */
+/* Theme CSS — unlayered so it overrides component layers (intentional).
+   Primitives set CSS custom properties, layer2 sets theme-aware rules,
+   semantic sets base styles on .themed containers. */
+@import "./theme-primitives.css";
+@import "./theme-layer2-attributes.css";
+@import "./theme-semantic.css";
 
 @import "./shop/components.css";
 @import "./shop/layout.css";