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/overrides.css

@@ -0,0 +1,7 @@
+/* Overrides — highest-priority layer for edge cases.
+   Theme editor preview frame rules go here (migrated from app-shop.css in Phase 5). */
+
+@layer overrides {
+  /* Cart drawer transitions (currently in app-shop.css, will move here) */
+  /* Preview-frame rules (currently in app.css, will move here) */
+}