Live wall attention cues: NEW lane, pin pulse, opt-in tone and notifications (#238)

- lib/attention.ts: pure snapshot diff (Major opened, severity escalated,
  pursuit path gained a stop, priority keyword in a new transcript) with a
  silent baseline on page open; calls baseline separately on their first
  non-empty snapshot. Diffed over all active incidents so widening a filter
  is never "new"; emitted, laned and pulsed only for the FILTERED view.
- components/live/NewLane.tsx: "NEW · last 15 min" overlay above the rail
  (desktop) / compact chip (mobile), Gate A notice included.
- components/live/AttentionControls.tsx: opt-in Web Audio tone (enabling
  click unlocks audio) and Web Notifications while the tab is hidden, body
  carries the machine-output notice; prefs + keywords in localStorage.
- LiveView wires it and passes pulseIncidentIds to MapView (2 min pulse).
- useIncidents: useActiveIncidentsState exposes a loaded flag (first
  server snapshot) so the baseline never sees an empty pre-load list.
- MachineOutputNotice exports MACHINE_OUTPUT_NOTICE_TEXT (same wording).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

drb-frontend/lib/useIncidents.ts

@@ -128,15 +128,24 @@ export function useIncident(incidentId: string | null) {
   return { incident, loading: loading || (!!incidentId && settledId !== incidentId) };
 }
 
-export function useActiveIncidents() {
+/**
+ * Active incidents plus whether the first SERVER snapshot has landed
+ * (server-26#238). `loaded` is what lets the wall tell "nothing active" from
+ * "not loaded yet" — attention cues baseline on it, so opening the page never
+ * fires a cue for every incident already open. Reset on sign-out / org change.
+ */
+export function useActiveIncidentsState(): { incidents: IncidentRecord[]; loaded: boolean } {
   const [incidents, setIncidents] = useState<IncidentRecord[]>([]);
+  const [loaded, setLoaded] = useState(false);
   const { orgId } = useAuth();
 
   useEffect(() => {
     let unsubFirestore: (() => void) | undefined;
+    setLoaded(false);
 
     const unsubAuth = onAuthStateChanged(auth, (user) => {
       if (unsubFirestore) { unsubFirestore(); unsubFirestore = undefined; }
+      setLoaded(false);
 
       if (!user) {
         setIncidents([]);
@@ -157,6 +166,7 @@ export function useActiveIncidents() {
             updated_at: toISO(data.updated_at),
           } as IncidentRecord;
         }));
+        if (!snap.metadata.fromCache) setLoaded(true);
       }, (err: FirestoreError) => { console.error("useActiveIncidents:", err); });
     });
 
@@ -166,5 +176,9 @@ export function useActiveIncidents() {
     };
   }, [orgId]);
 
-  return incidents;
+  return { incidents, loaded };
+}
+
+export function useActiveIncidents() {
+  return useActiveIncidentsState().incidents;
 }