add CSS migration foundation and screenshot tooling (Phase 0)

- CSS file structure with @layer declaration (reset, layout, components, utilities, overrides)
- Layout primitives: .stack, .cluster, .row, .auto-grid, .container-page, .with-sidebar, .center
- mix screenshots task using Playwright for visual regression testing
- Golden baseline captured (10 pages x 4 breakpoints = 40 screenshots)
- No visual changes — new CSS not wired into any layout yet

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

assets/css/shop.css

@@ -0,0 +1,17 @@
+/* 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. */
+
+@layer reset, primitives, tokens, components, layout, utilities, overrides;
+
+@import "./shop/reset.css";
+
+/* Theme primitives and tokens stay in their existing files.
+   They'll be wrapped in @layer during Phase 1. */
+
+@import "./shop/components.css";
+@import "./shop/layout.css";
+@import "./shop/utilities.css";
+@import "./shop/overrides.css";