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>

convex/products.ts

@@ -561,7 +561,24 @@ export const getByIdForAdmin = query({
   args: { id: v.id("products") },
   handler: async (ctx, { id }) => {
     await Users.requireAdmin(ctx);
-    return getProductWithRelations(ctx, id);
+    const product = await ctx.db.get(id);
+    if (!product) return null;
+
+    const [imagesRaw, variants, category] = await Promise.all([
+      ctx.db
+        .query("productImages")
+        .withIndex("by_product", (q) => q.eq("productId", id))
+        .collect(),
+      // Admin sees ALL variants (active and inactive)
+      ctx.db
+        .query("productVariants")
+        .withIndex("by_product", (q) => q.eq("productId", id))
+        .collect(),
+      ctx.db.get(product.categoryId),
+    ]);
+    const images = imagesRaw.sort((a, b) => a.position - b.position);
+
+    return { ...product, images, variants, category };
   },
 });
 
@@ -906,11 +923,20 @@ export const addVariant = mutation({
         v.literal("oz"),
       ),
     ),
+    length: v.optional(v.number()),
+    width: v.optional(v.number()),
+    height: v.optional(v.number()),
+    dimensionUnit: v.optional(v.union(v.literal("cm"), v.literal("in"))),
   },
   handler: async (ctx, args) => {
     await Users.requireAdmin(ctx);
     const product = await ctx.db.get(args.productId);
     if (!product) throw new Error("Product not found");
+    const existingSku = await ctx.db
+      .query("productVariants")
+      .withIndex("by_sku", (q) => q.eq("sku", args.sku))
+      .unique();
+    if (existingSku) throw new Error(`SKU "${args.sku}" is already in use`);
     return await ctx.db.insert("productVariants", {
       productId: args.productId,
       name: args.name,
@@ -922,6 +948,10 @@ export const addVariant = mutation({
       isActive: args.isActive,
       weight: args.weight ?? 0,
       weightUnit: args.weightUnit ?? "g",
+      ...(args.length !== undefined && { length: args.length }),
+      ...(args.width !== undefined && { width: args.width }),
+      ...(args.height !== undefined && { height: args.height }),
+      ...(args.dimensionUnit !== undefined && { dimensionUnit: args.dimensionUnit }),
     });
   },
 });
@@ -929,16 +959,45 @@ export const addVariant = mutation({
 export const updateVariant = mutation({
   args: {
     id: v.id("productVariants"),
+    name: v.optional(v.string()),
+    sku: v.optional(v.string()),
     price: v.optional(v.number()),
     compareAtPrice: v.optional(v.number()),
     stockQuantity: v.optional(v.number()),
     isActive: v.optional(v.boolean()),
+    weight: v.optional(v.number()),
+    weightUnit: v.optional(
+      v.union(
+        v.literal("g"),
+        v.literal("kg"),
+        v.literal("lb"),
+        v.literal("oz"),
+      ),
+    ),
+    attributes: variantAttributesValidator,
+    length: v.optional(v.number()),
+    width: v.optional(v.number()),
+    height: v.optional(v.number()),
+    dimensionUnit: v.optional(v.union(v.literal("cm"), v.literal("in"))),
   },
-  handler: async (ctx, { id, ...updates }) => {
+  handler: async (ctx, { id, sku, ...updates }) => {
     await Users.requireAdmin(ctx);
     const variant = await ctx.db.get(id);
     if (!variant) throw new Error("Variant not found");
+
+    // Enforce SKU uniqueness if sku is being changed
+    if (sku !== undefined && sku !== variant.sku) {
+      const existing = await ctx.db
+        .query("productVariants")
+        .withIndex("by_sku", (q) => q.eq("sku", sku))
+        .unique();
+      if (existing && existing._id !== id) {
+        throw new Error(`SKU "${sku}" is already in use by another variant`);
+      }
+    }
+
     const fields: Record<string, unknown> = {};
+    if (sku !== undefined) fields.sku = sku;
     for (const [key, value] of Object.entries(updates)) {
       if (value !== undefined) fields[key] = value;
     }