feat: render assistant replies as markdown

Add react-markdown + remark-math + rehype-katex to render the AI's
markdown (headings, bold, lists, code, blockquotes, tables) and LaTeX
math ($H_2O$ etc.). ReactMarkdown escapes raw HTML, keeping the stored
message rendering safe from XSS. Styled via .markdown-body in Message.css.
Applied to assistant messages only; user text stays plain.

frontend/package.json

@@ -9,8 +9,12 @@
     "preview": "vite preview"
   },
   "dependencies": {
+    "katex": "^0.18.4",
     "react": "^19.0.0",
-    "react-dom": "^19.0.0"
+    "react-dom": "^19.0.0",
+    "react-markdown": "^10.1.0",
+    "rehype-katex": "^7.0.1",
+    "remark-math": "^6.0.0"
   },
   "devDependencies": {
     "@vitejs/plugin-react": "^4.3.4",