feat(map): auto-fit the planner map to the trip's places on load

Closes the annoyance from discussion #510 — the planner opened every
trip centered on the global default, even when the trip's places were
on the other side of the world. We already have a BoundsController
that fits the map on the current places when fitKey changes, so
nudging fitKey once per trip (after the places have loaded) gives each
trip its own starting view without any new settings or UI. If a trip
has no places with coordinates yet, the global default still applies.

client/src/pages/TripPlannerPage.tsx

@@ -233,9 +233,19 @@ export default function TripPlannerPage(): React.ReactElement | null {
   const [showReservationModal, setShowReservationModal] = useState<boolean>(false)
   const [editingReservation, setEditingReservation] = useState<Reservation | null>(null)
   const [fitKey, setFitKey] = useState<number>(0)
+  const initialFitTripId = useRef<number | null>(null)
   const [mobileSidebarOpen, setMobileSidebarOpen] = useState<'left' | 'right' | null>(null)
   const [deletePlaceId, setDeletePlaceId] = useState<number | null>(null)
 
+  useEffect(() => {
+    if (!trip) return
+    if (initialFitTripId.current === trip.id) return
+    const hasGeoPlaces = places.some(p => p.lat != null && p.lng != null)
+    if (!hasGeoPlaces) return
+    initialFitTripId.current = trip.id
+    setFitKey(k => k + 1)
+  }, [trip, places])
+
   const connectionsStorageKey = tripId ? `trek:visible-connections:${tripId}` : null
   const [visibleConnections, setVisibleConnections] = useState<number[]>(() => {
     if (typeof window === 'undefined' || !connectionsStorageKey) return []