fix(offline): save packs after the page is open

[CodeDotJS]

Summary

• Open the app as soon as the script is ready. The loading scene covers that wait only.
• Save the offline files afterward: the app shell, then journey packs, then street tiles, two at a time and at low priority.
• A search while that save is running still loads that trip, including the fare, from the network.

Test plan

• Hard-refresh the preview and confirm the form appears without waiting for the journey files
• Confirm a route still shows a fare while the background save is running
• Confirm /snapshot/en/journeys/ requests start before /map-tiles/ requests