feat(eventOrder): add WhatsApp quote request functionality

Replaces the 'copy to clipboard' action with a direct 'Request a Quote' via WhatsApp. This streamlines the process for
users to submit their event order details.

- The 'Request a Quote' button now opens a pre-filled WhatsApp message to a designated contact number.
- A new `buildMessage` computed property is created in the `eventOrder` composable to generate a detailed quote request
message.
- Adds support for tracking referrers via a `?referer` URL query parameter, which is included in the final message.

app/components/eventOrder/OrderSummary.vue

@@ -58,19 +58,15 @@
 </template>
 
 <script lang="ts" setup>
-const { priceBreakdown, estimatedTotal, money } = useEventOrder();
+const { priceBreakdown, estimatedTotal, buildMessage, money } = useEventOrder();
 
 const items = computed(() => priceBreakdown.value);
 const total = computed(() => estimatedTotal.value);
 
 function onRequestQuote() {
-  // Placeholder interaction (no backend). Could open mailto or copy summary.
-  const lines = items.value.map(
-    (i) =>
-      `${i.label}：${money.format(i.amount)}${i.note ? `（${i.note}）` : ""}`
-  );
-  const text = `预算合计：${money.format(total.value)}\n\n` + lines.join("\n");
-  if (navigator?.clipboard) navigator.clipboard.writeText(text).catch(() => {});
+  const text = encodeURIComponent(buildMessage.value);
+  const url = `http://api.whatsapp.com/send?phone=+601157753558&text=${text}`;
+  window.open(url, "_blank");
 }
 </script>