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/public/dashboard.html

@@ -1334,6 +1334,11 @@ async function loadAlerts() {
     const r = await fetch('/api/dashboard/alerts');
     if (!r.ok) return;
     const data = await r.json();
+    if (data.noLocation) {
+      section.style.marginBottom = '1rem';
+      section.innerHTML = '<div class="alert-card sev-unknown" style="font-size:0.85rem">⚠ No location set — <a href="/dashboard/settings" style="color:var(--cool)">add your coordinates in Settings</a> to see NWS alerts.</div>';
+      return;
+    }
     const features = data.features || [];
     if (!features.length) { section.innerHTML = ''; section.style.marginBottom = '0'; return; }
     section.style.marginBottom = '1rem';