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/reset.css

@@ -0,0 +1,80 @@
+/* Reset — minimal, modern defaults
+   Normalises browser inconsistencies without being opinionated. */
+
+@layer reset {
+  *,
+  *::before,
+  *::after {
+    box-sizing: border-box;
+  }
+
+  * {
+    margin: 0;
+  }
+
+  html {
+    -webkit-text-size-adjust: 100%;
+    -moz-text-size-adjust: 100%;
+    text-size-adjust: 100%;
+  }
+
+  body {
+    line-height: 1.5;
+    -webkit-font-smoothing: antialiased;
+  }
+
+  img,
+  picture,
+  video,
+  canvas,
+  svg {
+    display: block;
+    max-width: 100%;
+  }
+
+  input,
+  button,
+  textarea,
+  select {
+    font: inherit;
+    color: inherit;
+  }
+
+  p,
+  h1,
+  h2,
+  h3,
+  h4,
+  h5,
+  h6 {
+    overflow-wrap: break-word;
+  }
+
+  a {
+    color: inherit;
+    text-decoration-skip-ink: auto;
+  }
+
+  ul,
+  ol {
+    list-style: none;
+    padding: 0;
+  }
+
+  fieldset {
+    border: none;
+    padding: 0;
+  }
+
+  table {
+    border-collapse: collapse;
+  }
+
+  /* Remove default button styling */
+  button {
+    background: none;
+    border: none;
+    padding: 0;
+    cursor: pointer;
+  }
+}