/observations/new: Show valid range for "Split Duration" field

🐞 Problem

When using the "Split Duration" option, the valid input range is not known to the user. When the user entered an invalid value it doesn't show an error but uses the default value of 720 seconds. This is confusing. By experimentation I found 121 seconds to be the minimum accepted value (which is useful for testing during station development).

💡 Possible Solution

Add a comment or tooltip which lists the accepted range, or alternatively fail on invalid inputs with an error message.