Autolink bare URLs in lesson comments

Comments render CommonMark, which leaves a pasted https://… as dead text —
students share links constantly and nobody writes [text](url) by hand. Add
remark-gfm so plain URLs, www hosts and emails become links (and tables and
strikethrough come along), plus table styling so a wide table scrolls inside
itself instead of stretching the page.

Sanitizing is unchanged: javascript: is neither linked nor autolinked, raw
HTML is still dropped, and third-party images are still rendered as plain
links by the img component.

package.json

@@ -49,6 +49,7 @@
     "react-dom": "19.2.4",
     "react-markdown": "^10.1.0",
     "rehype-sanitize": "^6.0.0",
+    "remark-gfm": "^4.0.1",
     "remark-parse": "^11.0.0",
     "resend": "^6.10.0",
     "sonner": "^2.0.7",