feat: status page shows effective refresh mode across all active clients

- Server tracks each client's refresh rate via query param on /user-downloads
- Active clients expire after 30s of no requests
- Status panel 'Data Refresh' card shows:
  - Background poll interval (or Disabled)
  - Effective mode: Background if all clients >= poll rate,
    Foreground (with rate) if any client is faster, Idle if no clients
  - Active client list with per-user refresh rate and last-seen age
- Foreground mode shown with orange badge for visibility
- Client refresh rate sent on every dashboard request

public/style.css

@@ -862,6 +862,25 @@ body {
   font-size: 0.7rem;
 }
 
+.status-fg-badge {
+  background: #fff3e0;
+  color: #e65100;
+  padding: 1px 8px;
+  border-radius: 8px;
+  font-size: 0.75rem;
+  font-weight: 600;
+}
+
+.status-row-sub {
+  padding-left: 12px;
+  font-size: 0.75rem;
+  opacity: 0.8;
+}
+
+.status-row-sub span:first-child {
+  font-style: italic;
+}
+
 .status-timings {
   display: flex;
   flex-direction: column;