fix(sync): re-hydrate active trip store on reconnect/online (H1) (#1181)

setRefetchCallback was dead code, so on reconnect the queue flushed and Dexie
re-seeded but the open trip's Zustand store was never refreshed — a
collaborator's edits made while we were offline didn't appear until navigating
away and back.

- new tripStore.hydrateActiveTrip(): silent refresh of the active trip's
  collaborative state (days/places/packing/todo/budget/reservations/files),
  no resetTrip and no isLoading toggle so there's no splash on reconnect
- syncTriggers wires setRefetchCallback to it (WS layer awaits the flush hook
  first) and re-hydrates open trips after the online-event syncAll; cleared on
  unregister
- websocket exposes getActiveTrips() for the online-event path
- tests: refetch wiring + ordering, silent hydrate without reset/splash

client/src/api/websocket.ts

@@ -20,6 +20,12 @@ export function getSocketId(): string | null {
   return mySocketId
 }
 
+/** Trip ids the app currently has open (joined). Used to re-hydrate the active
+ *  trip's store after the network comes back via the `online` event. */
+export function getActiveTrips(): string[] {
+  return Array.from(activeTrips)
+}
+
 export function setRefetchCallback(fn: RefetchCallback | null): void {
   refetchCallback = fn
 }