feat: add upgrade explainer

apps/web/src/views/checkout.ejs

@@ -119,6 +119,8 @@
       class="w-full btn-primary py-3">
       Continue to Payment
     </button>
+
+    <p class="text-xs text-gray-600 text-center mt-3">Upgrading? Your current plan's remaining time is frozen and resumes after the new plan expires. To merge existing time into a higher plan, contact support.</p>
   </form>
 
   <% } else { %>