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/app/globals.css

@@ -243,6 +243,24 @@
 }
 
 /* Markdown-разметка в комментариях под уроками (react-markdown) */
+/* Таблицы GFM: широкая таблица прокручивается внутри себя, а не растягивает страницу */
+.comment-md table {
+  display: block;
+  width: max-content;
+  max-width: 100%;
+  overflow-x: auto;
+  border-collapse: collapse;
+  margin: 0.5rem 0;
+  font-size: 0.95em;
+}
+.comment-md th,
+.comment-md td {
+  border: 1px solid var(--border);
+  padding: 0.35rem 0.6rem;
+  text-align: left;
+}
+.comment-md th { font-weight: 700; background: var(--color-surface); }
+.comment-md del { opacity: 0.6; }
 .comment-md img {
   display: block;
   max-width: 100%;