Plugin UI kit: stacked cards need air between them

`.wm-card` had no margin, so consecutive cards sat border-to-border and read as
one box — visible in antispam's settings (score / accounts / allow / refuse) and
in the filters plugin for the same reason.

Adjacent-sibling rather than a blanket margin: a card standing alone, like the
antispam banner inside a message, should not gain spacing its host did not ask
for.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018SqJYt7jD7oUEkoY68sBZo

frontend/src/plugins/plugin-ui.css

@@ -32,6 +32,12 @@
   border-radius: 8px;
   padding: 12px;
 }
+/* Stacked cards need air between them, or their borders sit flush and read as
+   one box. Adjacent-sibling so a card standing alone — the antispam banner
+   inside a message, say — gains no stray margin its host did not ask for. */
+.wm-card + .wm-card {
+  margin-top: 12px;
+}
 
 .wm-row {
   display: flex;