Redesign Upload Snapshot flow as a multi-step modal

### Description

The current Upload Snapshot flow (used on both Hosted and Remote Instances) is visually cluttered and hard to use. Redesign it as a multi-step flow:

- **Step 1:** Upload the snapshot file, with drag & drop support in addition to the regular file picker
- **Step 2 (final):** Configure name, description, flows, and environment variables

This should apply to both Hosted Instances and Remote Instances.

### Epic/Story

_No response_

### Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate