feat: Add rich template preview for WhatsApp & Twilio Templates (#13206)

This PR introduces a comprehensive, display-only template preview system
for both WhatsApp and Twilio templates rendering of all supported
template types.

This lays the foundation for rich template previews across:
- Template selection modals
- Campaign creation flows
- Message composer
- Message screen.

<img width="1818" height="2058" alt="template-preview"
src="https://github.com/user-attachments/assets/9833b28c-f824-4568-8f74-6da09ac61f62"
/>

---------

Co-authored-by: Sivin Varghese <64252451+iamsivin@users.noreply.github.com>
Co-authored-by: iamsivin <iamsivin@gmail.com>
Co-authored-by: Muhsin <12408980+muhsin-k@users.noreply.github.com>

app/javascript/dashboard/components-next/template-preview/TextTemplate.vue

@@ -0,0 +1,14 @@
+<script setup>
+defineProps({
+  message: {
+    type: Object,
+    required: true,
+  },
+});
+</script>
+
+<template>
+  <div class="p-3 rounded-xl bg-n-alpha-2 text-n-slate-12 max-w-80">
+    <span v-dompurify-html="message.content" class="prose prose-bubble" />
+  </div>
+</template>