Add upload mode selection with delete-absent-records option for Grade C dataset uploads

### StatGPT Admin Frontend version

0.0.0

### What is the problem this feature will solve?

Uploading a new Grade C dataset file always upserted records, with no way to also remove records that exist in the channel but are missing from the uploaded file. Users had no way to fully replace a dataset via upload.

### What is the feature you are proposing to solve the problem?

Add an upload mode selection to the Grade C dataset upload modal: after picking a file, show a "Also delete records not in this file" checkbox. Checking it uploads in "replace" mode (deletes absent records); leaving it unchecked keeps the existing "upsert" behavior.

### What alternatives have you considered?

N/A