fix(clients): unify client traffic chart timeframes to 2h/24h/7d and stop App Traffic card vanishing (v0.13.2-beta)

The two per-client usage charts exposed different time windows: "Wireless
Usage" offered 2h/24h/7d while "App Traffic (NetFlow)" offered 1h/24h/7d/30d.
Worse, selecting 1h (or any range with no flows in window) made the whole
App Traffic card unmount — including its range buttons, so it couldn't be
recovered without reloading.

- AppTrafficCard now uses 2h/24h/7d (default 24h), matching Wireless Usage
- Card visibility latches on first non-empty result, so switching to an
  empty range shows an inline "No traffic data yet for this range" state
  instead of unmounting the card (latch derived via guarded setState in
  render — no effect)
- Backend /api/traffic/* now accepts the '2h' range (rangeToFlux +
  windowForRange: 2h -> 2m buckets); previously '2h' silently fell back to 24h

Verified in the live stack with Playwright: ranges render as 2h/24h/7d (no
1h/30d), and clicking 2h on a client with data only outside the 2h window
keeps the card mounted with an empty-state and recovers on switching back.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/routes/trafficAnalytics.ts

@@ -8,7 +8,7 @@ const router = Router();
 router.use(requireAuth);
 
 function rangeToFlux(range: string): string {
-  const allowed = ['1h', '3h', '6h', '12h', '24h', '7d', '30d'];
+  const allowed = ['1h', '2h', '3h', '6h', '12h', '24h', '7d', '30d'];
   return allowed.includes(range) ? range : '24h';
 }
 
@@ -16,7 +16,7 @@ function rangeToFlux(range: string): string {
 // written every 60s by the collector flush).
 function windowForRange(range: string): string {
   const map: Record<string, string> = {
-    '1h': '1m', '3h': '5m', '6h': '5m', '12h': '10m',
+    '1h': '1m', '2h': '2m', '3h': '5m', '6h': '5m', '12h': '10m',
     '24h': '15m', '7d': '1h', '30d': '6h',
   };
   return map[range] || '15m';