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

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+/* 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";

assets/css/shop/components.css

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+/* Component styles — extracted from inline styles in later phases.
+   Each component gets its own section. */
+
+@layer components {
+  /* Phase 2: product cards, grid, badges, hero, categories */
+  /* Phase 2: PDP, variant selector, gallery, accordion */
+  /* Phase 3: layout components (header, footer, nav, search) */
+  /* Phase 3: cart components (drawer, items, summary) */
+  /* Phase 4: content components (contact, reviews, newsletter) */
+  /* Phase 4: page templates (checkout success, etc.) */
+}

assets/css/shop/layout.css

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+/* 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;
+  }
+}

assets/css/shop/overrides.css

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+/* 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) */
+}

assets/css/shop/reset.css

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+/* 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;
+  }
+}

assets/css/shop/utilities.css

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+/* Utility classes — a small, curated set for common patterns.
+   Not a framework. Just the handful that earn their keep. */
+
+@layer utilities {
+  .sr-only {
+    position: absolute;
+    width: 1px;
+    height: 1px;
+    padding: 0;
+    margin: -1px;
+    overflow: hidden;
+    clip: rect(0, 0, 0, 0);
+    white-space: nowrap;
+    border-width: 0;
+  }
+
+  .truncate {
+    overflow: hidden;
+    text-overflow: ellipsis;
+    white-space: nowrap;
+  }
+
+  .text-balance {
+    text-wrap: balance;
+  }
+
+  /* Hide visually but keep in DOM (for phx-update="stream" empty states etc.) */
+  .visually-hidden:not(:focus):not(:active) {
+    position: absolute;
+    width: 1px;
+    height: 1px;
+    padding: 0;
+    margin: -1px;
+    overflow: hidden;
+    clip: rect(0, 0, 0, 0);
+    white-space: nowrap;
+    border-width: 0;
+  }
+}