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/layout.css

@@ -0,0 +1,69 @@
+/* Layout primitives — composable building blocks for page structure.
+   Each primitive does one layout job well. Combine them freely. */
+
+@layer layout {
+  /* Vertical stack with consistent gap */
+  .stack {
+    display: flex;
+    flex-direction: column;
+    gap: var(--stack-gap, var(--space-md));
+  }
+
+  /* Horizontal flex-wrap cluster for tags, pills, badges */
+  .cluster {
+    display: flex;
+    flex-wrap: wrap;
+    gap: var(--cluster-gap, var(--space-sm));
+    align-items: center;
+  }
+
+  /* Horizontal flex row, no wrap — navbars, toolbars, inline groups */
+  .row {
+    display: flex;
+    align-items: center;
+    gap: var(--row-gap, var(--space-sm));
+  }
+
+  /* Intrinsic responsive grid — items wrap naturally without breakpoints */
+  .auto-grid {
+    display: grid;
+    grid-template-columns: repeat(
+      auto-fill,
+      minmax(min(var(--auto-grid-min, 16rem), 100%), 1fr)
+    );
+    gap: var(--auto-grid-gap, var(--space-md));
+  }
+
+  /* Centered max-width container */
+  .container-page {
+    width: 100%;
+    max-width: var(--t-layout-max-width, 1400px);
+    margin-inline: auto;
+    padding-inline: var(--space-md);
+  }
+
+  /* Main content + sidebar layout */
+  .with-sidebar {
+    display: flex;
+    flex-wrap: wrap;
+    gap: var(--sidebar-gap, var(--space-xl));
+
+    & > :first-child {
+      flex-grow: 999;
+      flex-basis: 0;
+      min-width: 60%;
+    }
+
+    & > :last-child {
+      flex-grow: 1;
+      flex-basis: var(--sidebar-width, 20rem);
+    }
+  }
+
+  /* Flex center — for centering content both axes */
+  .center {
+    display: flex;
+    align-items: center;
+    justify-content: center;
+  }
+}