chore: upgrade @muyajs/core to 0.1.0 and drop mermaid workaround (#99)

Muya 0.1.0 fixes the walkTokens accumulation bug that wiped code-block
languages on the 2nd+ call to MarkdownToHtml.renderHtml() and also
renders mermaid SVGs internally. Remove the rewrapMermaidSource fallback
and the manual mermaid passes in Feature/Theme, and drop the now-unused
mermaid dependency.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Jocs

package.json

@@ -11,9 +11,8 @@
     "type-check": "tsc --noEmit"
   },
   "dependencies": {
-    "@muyajs/core": "^0.0.39",
+    "@muyajs/core": "^0.1.0",
     "katex": "^0.16.9",
-    "mermaid": "^11.12.1",
     "react": "^18.2.0",
     "react-dom": "^18.2.0"
   },