v3: error cards across the app, friendly network errors, code quality pass

src/components/CommentThread.tsx

@@ -6,6 +6,7 @@ import { deserializeComment } from "../model.ts";
 import { Avatar } from "./Avatar.tsx";
 import { Markdown } from "./Markdown.tsx";
 import { relativeTime } from "../utils/relativeTime.ts";
+import { ErrorCard } from "./ErrorCard.tsx";
 
 interface CommentThreadProps {
   dumpId: string;
@@ -204,7 +205,9 @@ function CommentNode({
                 placeholder="Write a reply…"
                 rows={3}
               />
-              {replyError && <p className="comment-form-error">{replyError}</p>}
+              {replyError && (
+                <ErrorCard title="Failed to post reply" message={replyError} />
+              )}
               <div className="comment-form-actions">
                 <button
                   type="submit"
@@ -323,7 +326,7 @@ export function CommentThread({
             rows={3}
           />
           {topLevelError && (
-            <p className="comment-form-error">{topLevelError}</p>
+            <ErrorCard title="Failed to post comment" message={topLevelError} />
           )}
           <div className="comment-form-actions">
             <button