fix(sidebar): remove 1-hour staleness filter from activity ticker (#786)

Show the most recent stack event regardless of age. The ticker now
stays populated as long as any event arrived in the current session,
with formatTimeAgo reflecting the true elapsed time (e.g. '2h ago').
Idle state only appears when no events have arrived yet.

Also reduces the update interval from 30s to 10s so relative timestamps
stay responsive.

frontend/src/components/sidebar/SidebarActivityTicker.tsx

@@ -1,10 +1,9 @@
-import { useEffect, useMemo, useState } from 'react';
+import { useEffect, useMemo, useReducer } from 'react';
 import { cn } from '@/lib/utils';
 import { formatTimeAgo } from '@/lib/relativeTime';
 import type { NotificationItem } from '@/components/dashboard/types';
 
-const ONE_HOUR_S = 60 * 60;
-const NOW_TICK_MS = 30_000;
+const NOW_TICK_MS = 10_000;
 
 interface SidebarActivityTickerProps {
   notifications: NotificationItem[];
@@ -13,16 +12,16 @@ interface SidebarActivityTickerProps {
 }
 
 export function SidebarActivityTicker({ notifications, connected, onNavigate }: SidebarActivityTickerProps) {
-  const [nowS, setNowS] = useState(() => Math.floor(Date.now() / 1000));
+  const [, forceUpdate] = useReducer((x: number) => x + 1, 0);
   useEffect(() => {
-    const id = setInterval(() => setNowS(Math.floor(Date.now() / 1000)), NOW_TICK_MS);
+    const id = setInterval(forceUpdate, NOW_TICK_MS);
     return () => clearInterval(id);
   }, []);
   const latest = useMemo(() => {
     return notifications
-      .filter(n => n.stack_name && nowS - n.timestamp <= ONE_HOUR_S)
+      .filter(n => n.stack_name)
       .sort((a, b) => b.timestamp - a.timestamp)[0] ?? null;
-  }, [notifications, nowS]);
+  }, [notifications]);
 
   const idle = latest === null;
   const dotClass = connected