feat(renderer): Mermaid diagram rendering with dark theme (M10.7)

- Lazy-loads Mermaid.js on first ```mermaid block detected
- Dark theme with Bitcoin orange accent colors
- Cached renders to avoid re-rendering on scroll
- Error display inline without crashing app

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

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

@@ -326,6 +326,7 @@
 import { computed, ref, nextTick, watch } from 'vue'
 import MarkdownIt from 'markdown-it'
 import { hasMath, renderMathInHtml } from '@/composables/useMathRenderer'
+import { hasMermaid, renderMermaidBlocks } from '@/composables/useMermaidRenderer'
 import ContextMenu from '@/components/ui/ContextMenu.vue'
 import ContextMenuItem from '@/components/ui/ContextMenuItem.vue'
 import type { Message, WebSearchResult } from '@aiui/core/types/message'
@@ -497,14 +498,17 @@ const displayText = computed(() => {
 const baseMarkdown = computed(() => md.render(displayText.value))
 const renderedMarkdown = ref('')
 
-// Render math after markdown, batch on content change
+// Render math/mermaid after markdown, batch on content change
 watch(baseMarkdown, async (html) => {
+  let result = html
+  renderedMarkdown.value = html // Show immediately
   if (hasMath(displayText.value)) {
-    renderedMarkdown.value = html // Show immediately
-    renderedMarkdown.value = await renderMathInHtml(html)
-  } else {
-    renderedMarkdown.value = html
+    result = await renderMathInHtml(result)
   }
+  if (hasMermaid(displayText.value)) {
+    result = await renderMermaidBlocks(result)
+  }
+  renderedMarkdown.value = result
 }, { immediate: true })
 
 const formattedTime = computed(() => {