feat(renderer): KaTeX math rendering for $inline$ and $$block$$ (M10.6)

- Lazy-loads KaTeX (~70KB) on first math detected
- Inline $...$ and block $$...$$ LaTeX support
- Falls back to <code> for invalid LaTeX
- Batch render after markdown, not during streaming

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/app/src/components/chat/ChatMessage.vue

@@ -323,8 +323,9 @@
 </template>
 
 <script setup lang="ts">
-import { computed, ref, nextTick } from 'vue'
+import { computed, ref, nextTick, watch } from 'vue'
 import MarkdownIt from 'markdown-it'
+import { hasMath, renderMathInHtml } from '@/composables/useMathRenderer'
 import ContextMenu from '@/components/ui/ContextMenu.vue'
 import ContextMenuItem from '@/components/ui/ContextMenuItem.vue'
 import type { Message, WebSearchResult } from '@aiui/core/types/message'
@@ -493,9 +494,18 @@ const displayText = computed(() => {
   return text
 })
 
-const renderedMarkdown = computed(() => {
-  return md.render(displayText.value)
-})
+const baseMarkdown = computed(() => md.render(displayText.value))
+const renderedMarkdown = ref('')
+
+// Render math after markdown, batch on content change
+watch(baseMarkdown, async (html) => {
+  if (hasMath(displayText.value)) {
+    renderedMarkdown.value = html // Show immediately
+    renderedMarkdown.value = await renderMathInHtml(html)
+  } else {
+    renderedMarkdown.value = html
+  }
+}, { immediate: true })
 
 const formattedTime = computed(() => {
   const d = new Date(props.message.timestamp)