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.

src/components/student/lesson-comments.tsx

@@ -2,6 +2,7 @@
 
 import { useRef, useState, useTransition, type ReactNode } from "react";
 import ReactMarkdown from "react-markdown";
+import remarkGfm from "remark-gfm";
 import rehypeSanitize, { defaultSchema } from "rehype-sanitize";
 import { addComment, deleteComment, editComment } from "@/lib/actions/student-actions";
 
@@ -63,6 +64,9 @@ function CommentBody({ text, imageHost }: { text: string; imageHost: string }) {
   return (
     <div className="comment-md text-sm leading-relaxed break-words">
       <ReactMarkdown
+        // GFM: голый адрес сам становится ссылкой. Без него студент пишет
+        // https://… и получает мёртвый текст — разметку никто не проставляет.
+        remarkPlugins={[remarkGfm]}
         rehypePlugins={[[rehypeSanitize, sanitizeSchema]]}
         components={{
           // eslint-disable-next-line @typescript-eslint/no-unused-vars -- отбрасываем служебный node от react-markdown, чтобы он не протекал в DOM