Support "route not found" views with the FS router

[Legioth]

Describe your motivation

There's no way of defining a client-side catch-all route that would be used in case there isn't any specific matching route. This is handled through ...serverSideRoutes when there are Flow views in the application but the plan is to refrain from delegating to the server if there are no server-side routes.

Describe the solution you'd like

A file named $404.tsx inside frontend/views/ should be used as a * fallback handler on the corresponding level in the generated router configuration.

Describe alternatives you've considered

The alternative is to manually wrap the routing configuration from views.ts to add an outer layer with a * fallback route.

Additional context

No response