feat(feed): handle followers-only posts with locked UI (issue #246)

frontend/app/globals.css

@@ -784,3 +784,8 @@ select:disabled {
   padding-top: var(--space-sm, 10px);
   border-top: 1px solid var(--color-border, rgba(0,0,0,0.08));
 }
+.post-locked-hint {
+  margin-top: 0.5rem;
+  font-size: 0.85rem;
+  opacity: 0.7;
+}