feat(admin): implement variant management — list, create, edit, preview, activate/deactivate, delete (Plan 05)

- Extend addVariant with dimension fields and SKU uniqueness check; expand updateVariant to full field set; update getByIdForAdmin to return all variants (active + inactive)
- Add generateSku utility to @repo/utils; auto-generates SKU from brand, product name, attributes, and weight with manual-override support
- Move ProductSearchSection to components/shared and fix nav link /variants → /variant
- Variants page: product search, loading skeleton, variants table, toolbar with create button
- VariantsTable: 8 columns, activate/deactivate toggle, delete with AlertDialog confirmation
- VariantPreviewDialog: read-only full variant details with sections for pricing, inventory, shipping, attributes
- VariantForm: zod schema with superRefine for dimension and on-sale validation, auto-SKU generation
- CreateVariantDialog and EditVariantDialog wiring dollarsToCents on submit
- Install sonner and add Toaster to root layout; install ShadCN Switch component

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package-lock.json

@@ -63,6 +63,7 @@
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         "zod": "^3.25.76"
       },
@@ -16611,6 +16612,16 @@
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       "license": "MIT"
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+    "node_modules/sonner": {
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+      "license": "MIT",
+      "peerDependencies": {
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+        "react-dom": "^18.0.0 || ^19.0.0 || ^19.0.0-rc"
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