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/templates/settings.html

@@ -301,7 +301,7 @@
 <div id="sync-modal" class="modal"
      style="position:fixed;inset:0;background:rgba(0,0,0,0.45);
             display:none;align-items:center;justify-content:center;z-index:1000;">
-  <div style="background:var(--panel-bg,#fff);color:var(--text,#000);
+  <div style="background:var(--surface);color:var(--text);
               padding:22px 26px;border-radius:8px;max-width:440px;width:90%;">
     <div class="result__head" id="sync-modal-title" style="margin-bottom:8px;">
       Enable cloud sync
@@ -355,7 +355,7 @@
     const errEl = $('sync-modal-err');
 
     function setFeedback(msg, ok) {
-      feedbackEl.style.color = ok ? 'var(--ok,#2a9d57)' : '';
+      feedbackEl.style.color = ok ? 'var(--positive)' : '';
       feedbackEl.textContent = msg || '';
     }
     // External callers (the Import section above) can pass a callback