Fix undefined values in lake CSV filenames

**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: `____.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**

Start at the CSV download flow and locate the filename construction that uses featureType, riverName, reachId, startTime, and endTime. Reproduce a lake download at swotviz.cuahsi.io, then verify the filename uses the lake name and lake ID while river filenames remain unchanged.

Written by the indexing model from the issue text.