feat: add Copy Text and Download .docx buttons to response window

- Copy Text: copies raw markdown to clipboard with visual feedback
- Download .docx: converts markdown to a properly formatted Word
  document using the docx library (headings, bold/italic, lists,
  tables, blockquotes, links, horizontal rules)
- Both buttons appear below the response article after streaming ends
- docx/file-saver are lazy-loaded (dynamic import) so they don't
  bloat the initial bundle (~308 KB main vs ~407 KB docx chunk)

Co-Authored-By: alex <alex@semipublic.co>

package.json

@@ -10,6 +10,8 @@
     "preview": "vite preview"
   },
   "dependencies": {
+    "docx": "^9.6.1",
+    "file-saver": "^2.0.5",
     "react": "^18.3.1",
     "react-dom": "^18.3.1",
     "react-markdown": "^9.0.1",