[ACC-005] Accounts: Wishlist — save and manage favorite products

## Overview
Let logged-in users save products to a wishlist for later. Wishlist items should be visible on the product grid and accessible from the user account page.

## Acceptance Criteria
- [ ] Heart/bookmark icon on each product card toggles wishlist status
- [ ] `POST /api/wishlist/:productId` — add to wishlist
- [ ] `DELETE /api/wishlist/:productId` — remove from wishlist
- [ ] `GET /api/wishlist` — returns user's wishlist
- [ ] "My Wishlist" section on the account page (#10) shows saved items
- [ ] "Move to Cart" button on wishlist items calls cart add (#3)
- [ ] Wishlist state is not available to guests (prompt to sign in)
- [ ] Wishlist count shown in the header (badge on heart icon)

## Depends On
- #5 AUTH-001 — user identity required
- #3 CART-003 — "Move to Cart" action depends on cart API

## Blocks
- #10 ACC-006 (wishlist section rendered within profile page)