Avoid missing image if json image list is empty

[sirtet]

If the webcam stops delivering images for a few days, the UI will show no cam image, as on initial load a default range back from the current date is requested.

I see two options to avoid having no image:

1. Do not request a default range, api.php has a default in case no start- and end- date is requested.
   To make it obvious to the user that the received pictures are possibly outdated, a date check should be done and a message similar to the suggestion in 2.
2. If the answer from api.php is empty, show a message "no current images available, loading (trying to load) last available images"
   (and maybe do an api call without time range now, to show the last available pics.)

[sirtet]

As a temporary fix, i changed api.php so it returns the last available images if the requested range is too new.
But this will create a discrepancy in shown date-range vs. the dropdowns for requested range.

[sirtet]

Idea: The Frontend should show the latest available image and alert (overlay) the user about the fact that the requested Range has no images.