fix: resolve JS errors on messages and platform-homepage pages

- messages.js: Add missing `error` property required by error_state macro
- platform-homepage.html: Use `<template x-if>` instead of `x-show` to
  prevent Alpine from evaluating `page.template` when `page` is null

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

static/admin/js/messages.js

@@ -20,6 +20,7 @@ function adminMessages(initialConversationId = null) {
 
         // Loading states
         loading: true,
+        error: null,
         loadingConversations: false,
         loadingMessages: false,
         sendingMessage: false,