feat(frontend): add product detail view and navigation

- Create ProductView component with product details display
- Add ProductView.css with responsive styling
- Add product screen to App.jsx screen system
- Pass onNavigateToProduct callback from App to SearchView
- Update ProductResults onSelect to use callback instead of window.location

apps/frontend/src/views/ProductView.css

@@ -0,0 +1,102 @@
+.product-view {
+  width: 100%;
+  max-width: 48rem;
+  margin: 0 auto;
+  padding: 1rem var(--margin-main) 2rem;
+  animation: fadeInUp 0.3s ease-out;
+}
+
+.back-btn {
+  display: flex;
+  align-items: center;
+  gap: 0.5rem;
+  padding: 0.5rem 0;
+  margin-bottom: 1rem;
+  background: none;
+  border: none;
+  color: var(--primary, #2563eb);
+  font-size: 0.875rem;
+  font-weight: 500;
+  cursor: pointer;
+}
+
+.back-btn:hover {
+  opacity: 0.8;
+}
+
+.product-loading,
+.product-error {
+  text-align: center;
+  padding: 3rem 1rem;
+  color: var(--on-surface-variant, #6b7280);
+  font-size: 1rem;
+}
+
+.product-header {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  margin-bottom: 1.5rem;
+}
+
+.product-image {
+  width: 120px;
+  height: 120px;
+  object-fit: contain;
+  border-radius: var(--radius-md, 0.75rem);
+  margin-bottom: 1rem;
+}
+
+.source-badge {
+  display: inline-block;
+  padding: 0.25rem 0.75rem;
+  border-radius: 9999px;
+  color: white;
+  font-size: 0.75rem;
+  font-weight: 600;
+}
+
+.product-name {
+  font-size: 1.5rem;
+  font-weight: 700;
+  color: var(--on-surface);
+  text-align: center;
+  margin-bottom: 0.25rem;
+}
+
+.product-brand {
+  font-size: 1rem;
+  color: var(--on-surface-variant, #6b7280);
+  text-align: center;
+  margin-bottom: 1.5rem;
+}
+
+.product-details {
+  background: var(--surface-container-lowest, #f9fafb);
+  border-radius: var(--radius-md, 0.75rem);
+  padding: 1rem;
+}
+
+.detail-row {
+  display: flex;
+  flex-direction: column;
+  gap: 0.25rem;
+  padding: 0.75rem 0;
+  border-bottom: 1px solid var(--outline-variant, #e5e7eb);
+}
+
+.detail-row:last-child {
+  border-bottom: none;
+}
+
+.detail-label {
+  font-size: 0.75rem;
+  font-weight: 600;
+  color: var(--on-surface-variant, #9ca3af);
+  text-transform: uppercase;
+}
+
+.detail-value {
+  font-size: 0.9375rem;
+  color: var(--on-surface);
+}