feat: full UI redesign with design system, Nielsen heuristics compliance

- Install lucide-react, framer-motion, react-hot-toast, clsx, tailwind-merge
- Custom Tailwind config: semantic color tokens, Inter font, shadow scale,
  border radius scale, custom animations (fadeIn, slideUp, shimmer)
- Shared component library: Button, Badge, Card, Input, Select, Textarea,
  EmptyState, Skeleton, LoadingSpinner
- Global CSS with @layer components (.btn, .card, .input, .badge, .skeleton)
- Toast notification system via react-hot-toast + showToast utility
- ErrorBoundary wrapper for graceful error recovery
- Redesigned navigation: sticky, active state indicators, Lucide icons
- Dashboard: hero header, today highlighting, scrollable week grid,
  redesigned meal cards, empty states, skeleton loading
- Meal Detail: hero image with gradient overlay, metadata row with icons,
  Lucide star rating, edit-existing-feedback flow
- Pantry: inline add form, search/filter, visual quantity badges,
  expiry warnings, confirmation dialogs
- Shopping List: gradient summary cards, aisle grouping with badges,
  sale strikethrough pricing, empty state
- Login: centered card with icon, Input component, Button component
- All old gray/blue utility classes migrated to new surface/primary tokens
- TypeScript clean, production build passes

frontend/src/index.css

@@ -1,10 +1,77 @@
+@import url('https://fonts.googleapis.com/css2?family=Inter:wght@300;400;500;600;700;800&display=swap');
+
 @tailwind base;
 @tailwind components;
 @tailwind utilities;
 
-body {
-  margin: 0;
-  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen, Ubuntu, Cantarell, 'Open Sans', 'Helvetica Neue', sans-serif;
-  -webkit-font-smoothing: antialiased;
-  -moz-osx-font-smoothing: grayscale;
+@layer base {
+  html {
+    -webkit-font-smoothing: antialiased;
+    -moz-osx-font-smoothing: grayscale;
+    text-rendering: optimizeLegibility;
+  }
+
+  body {
+    margin: 0;
+    font-family: 'Inter', system-ui, -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
+    background-color: #f5f5f5;
+    color: #171717;
+    line-height: 1.5;
+  }
+
+  * {
+    border-color: #e5e5e5;
+  }
+
+  :focus-visible {
+    outline: 2px solid #0ea5e9;
+    outline-offset: 2px;
+  }
+
+  ::selection {
+    background-color: #bae6fd;
+    color: #0c4a6e;
+  }
+}
+
+@layer components {
+  .btn {
+    @apply inline-flex items-center justify-center gap-2 px-4 py-2.5 rounded-lg font-medium text-sm transition-all duration-200 focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-primary-500 focus-visible:ring-offset-2 disabled:opacity-50 disabled:cursor-not-allowed;
+  }
+
+  .btn-primary {
+    @apply btn bg-primary-600 text-white hover:bg-primary-700 active:bg-primary-800 shadow-sm;
+  }
+
+  .btn-secondary {
+    @apply btn bg-white text-surface-700 border border-surface-300 hover:bg-surface-50 active:bg-surface-100;
+  }
+
+  .btn-ghost {
+    @apply btn text-surface-600 hover:bg-surface-100 active:bg-surface-200;
+  }
+
+  .btn-danger {
+    @apply btn bg-danger-600 text-white hover:bg-danger-700 active:bg-danger-800;
+  }
+
+  .card {
+    @apply bg-white rounded-xl shadow-sm border border-surface-200 overflow-hidden;
+  }
+
+  .input {
+    @apply w-full px-3 py-2.5 rounded-lg border border-surface-300 bg-white text-sm text-surface-900 placeholder:text-surface-400 transition-all duration-200 focus:border-primary-500 focus:ring-2 focus:ring-primary-100 focus:outline-none disabled:opacity-50 disabled:cursor-not-allowed;
+  }
+
+  .label {
+    @apply block text-sm font-medium text-surface-700 mb-1.5;
+  }
+
+  .badge {
+    @apply inline-flex items-center gap-1 px-2.5 py-0.5 rounded-full text-xs font-medium;
+  }
+
+  .skeleton {
+    @apply animate-shimmer bg-gradient-to-r from-surface-200 via-surface-300 to-surface-200 bg-[length:200%_100%] rounded-md;
+  }
 }