ui: fix chat pending class, invented CSS vars, .pf-secondary scope

- chat.js: pending indicator class was wrong (.pending instead of
  chat-msg--pending) so the … waiting message never got italic/dim
- settings.html + cassandra.css: three invented CSS vars (--panel-bg,
  --ok, --surface-1) had hardcoded fallbacks that broke dark mode;
  replaced with real tokens (--surface, --positive)
- cassandra.css: .pf-secondary was scoped to .pf-actions but used
  standalone in 4 places (sync modal, disable-sync, import cancel,
  forget-pie button) — hoisted to a top-level selector

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/static/js/chat.js

@@ -37,7 +37,8 @@
     append('user', text);
     input.value = '';
     send.disabled = true;
-    const thinking = append('assistant pending', '…');
+    const thinking = append('assistant', '…');
+    thinking.classList.add('chat-msg--pending');
     try {
       const r = await fetch('/api/chat', {
         method: 'POST',