Allow image uploads in lesson comments

Add an image button to the comment toolbar backed by a new upload route
(images only, 5 MB, stored under comments/<userId>/), inserted as Markdown.

Only images served from our own storage are rendered: a third-party src would
be fetched by every reader of the lesson, exposing their address. Such links
are shown as plain links instead — this also defuses the external images a few
existing comments already carry.

src/app/globals.css

@@ -243,6 +243,13 @@
 }
 
 /* Markdown-разметка в комментариях под уроками (react-markdown) */
+.comment-md img {
+  display: block;
+  max-width: 100%;
+  height: auto;
+  margin: 0.5rem 0;
+  border: 2px solid var(--border);
+}
 .comment-md > :first-child { margin-top: 0; }
 .comment-md > :last-child { margin-bottom: 0; }
 .comment-md p { margin: 0 0 0.5rem; }