Fix read-only mode: hide write actions when no signer

- ComposeBox, action row (reply/like/zap), and write article button
  all gated on getNDK().signer in addition to loggedIn
- Prevents read-only (npub-only) users from seeing actions they can't use
- Found during browser testing of the login path

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/components/feed/NoteCard.tsx

@@ -5,7 +5,7 @@ import { useReactionCount } from "../../hooks/useReactionCount";
 import { useUserStore } from "../../stores/user";
 import { useUIStore } from "../../stores/ui";
 import { timeAgo, shortenPubkey } from "../../lib/utils";
-import { publishReaction, publishReply } from "../../lib/nostr";
+import { publishReaction, publishReply, getNDK } from "../../lib/nostr";
 import { NoteContent } from "./NoteContent";
 import { ZapModal } from "../zap/ZapModal";
 
@@ -122,7 +122,7 @@ export function NoteCard({ event }: NoteCardProps) {
           </div>
 
           {/* Actions */}
-          {loggedIn && (
+          {loggedIn && !!getNDK().signer && (
             <div className="flex items-center gap-4 mt-2">
               <button
                 onClick={handleReply}