Add discovery datasets / glossary import mode switches to channel import popup

### StatGPT Admin Frontend version

0.0.0 (current development)

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

The channel import flow always upserts discovery datasets and glossary terms into an existing channel — there's no way to instead replace those collections wholesale from the imported archive, so stale records the archive no longer carries are left behind.

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

Add two mode switches to the import channel popup, one for discovery datasets and one for glossary terms, each toggling between "upsert" (default, current behavior) and "replace". The discovery datasets switch is Grade C functionality, so it must be gated behind the `ENABLE_DISCOVERY_DATASETS` feature toggle like the rest of the Grade C surface; the glossary terms switch is unaffected by that toggle.

### What alternatives have you considered?

Always defaulting to "replace" — rejected because it would silently delete curated records on every import when the archive is missing a collection, which is a behavior change from what admins expect today.

### References

Related backend issue: epam/statgpt-backend#653
See implementation details: epam/statgpt-backend#657