Fix NWS alerts not showing when no location is configured

- Show a prompt linking to Settings instead of silently hiding the
  alerts section when no lat/lon is saved
- Clear the alert cache when location is saved so alerts appear
  immediately without waiting for the 5-minute cache to expire

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/server.js

@@ -1344,6 +1344,7 @@ app.post('/api/dashboard/location', requireDashboardAuth, (req, res) => {
     return res.status(400).json({ error: 'Invalid coordinates' });
   dashAuth = { ...dashAuth, stationLat: +lat, stationLon: +lon };
   try { fs.writeFileSync(DASH_AUTH_FILE, JSON.stringify(dashAuth)); } catch {}
+  dashAlertCache.clear();
   res.json({ ok: true });
 });