feat: Add cart store for catalog quantity sync

- Create Pinia cart store to manage cart state globally
- Sync quantity changes between catalog cards and cart sidebar
- Improve product card design with borders and hover effects
- Fix cart sync when updating quantity via minus button or input field

src/stores/cart.js

@@ -0,0 +1,43 @@
+import { defineStore } from 'pinia'
+
+export const useCartStore = defineStore('cart', {
+  state: () => ({
+    items: []
+  }),
+  getters: {
+    cartCount: (state) => state.items.reduce((sum, item) => sum + item.quantity, 0),
+    cartTotal: (state) => state.items.reduce((sum, item) => sum + (item.price * item.quantity), 0)
+  },
+  actions: {
+    addItem(product) {
+      const existing = this.items.find(i => i.id === product.id)
+      if (existing) {
+        existing.quantity = product.quantity
+      } else {
+        this.items.push({
+          id: product.id,
+          name: product.name,
+          img: product.img,
+          price: product.price,
+          quantity: product.quantity,
+          measuring_unit: product.measuring_unit || 'Unidad'
+        })
+      }
+    },
+    removeItem(itemId) {
+      const index = this.items.findIndex(i => i.id === itemId)
+      if (index > -1) {
+        this.items.splice(index, 1)
+      }
+    },
+    updateQuantity({ itemId, quantity }) {
+      const item = this.items.find(i => i.id === itemId)
+      if (item) {
+        item.quantity = quantity
+      }
+    },
+    clearCart() {
+      this.items = []
+    }
+  }
+})