Fix undefined values in lake CSV filenames

[igarousi]

Describe the bug
CSV filenames currently use the same naming convention for both rivers and lake data.
filename = ${featureType}_${riverName}_${reachId}_${startTime}_${endTime}

This works for river data, but lake features do not have rivername or reachid fields. As a result, lake downloads can produce filenames such as: priorlake_undefined_undefined_2023_03_01t00_00_00z_2026_08_11t16_34_28z.csv

For your reference: Lake support was added after the original river-data implementation, but the filename construction logic was not updated to account for lake-specific fields.

To Reproduce
Steps to reproduce the behavior:

1. Go to https://swotviz.cuahsi.io
2. Click on a lake
3. Generate a plot
4. Download the data as a CSV

Expected behavior
The filename for lake downloads should be: <featureType>_<lakeName>_<lakeId>_<startTime>_<endTime>.csv
The lake name is already included in the downloaded csv. The lake ID is available in the app's metadata table after a lake is selected.

Screenshots