[App] Easier Grade Filtering in Manage People

Overview When filtering people by grade in the Manage People section, you now see a dropdown list of all available grades instead of having to type them in manually!
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Simpler grade selection The advanced filter in Manage People now shows you all the grade levels that exist in your system. Just click the dropdown and select the grades you want to filter by - no more guessing at grade formats or trying to remember exact grade names.
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Consistent with target lists This works the same way as the grade filter we recently improved in target lists, giving you a consistent experience when working with grade-based filtering throughout the platform.
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Reduces filtering mistakes By showing you exactly which grades are available, you can be confident you're selecting the right criteria and won't miss people due to typing errors or incorrect grade formats.

This small improvement makes it much easier to find and manage people by grade level in your system.