feat(notifications): coalesce identical notifications with a repeat count

Fold a burst of identical notifications (same level+title+message within a
short window) into one entry with a ×N count instead of stacking copies — tames
a crash-retry loop or a repeatedly-failing poll. Shown in the toast and centre.

frontend/src/components/NotificationsPanel.tsx

@@ -320,7 +320,14 @@ function ClientActivityRow({
       <Icon className={`w-4 h-4 shrink-0 mt-0.5 ${color}`} />
       <div className="min-w-0 flex-1">
         {n.title && <div className="text-sm font-semibold">{n.title}</div>}
-        <div className="text-sm break-words">{n.message}</div>
+        <div className="text-sm break-words">
+          {n.message}
+          {n.repeat > 1 && (
+            <span className="ml-1.5 align-middle text-[11px] font-semibold text-muted tabular-nums">
+              ×{n.repeat}
+            </span>
+          )}
+        </div>
         <div className="flex items-center gap-2 mt-0.5">
           <span className="text-[11px] text-muted">{relativeFromMs(n.ts)}</span>
           {needsAction && (