feat: add pre-upload cost estimation

## Description

Users have no way to estimate storage costs before uploading or committing a deposit. The `payments setup` flow shows an example deposit amount but expects users to do their own math. There is no way to ask "how much do I need to store X GiB for Y months?" before funds are committed. This creates friction at the most sensitive moment of onboarding, first deposit, and forces users to guess or over-deposit.

## Proposed solution

## Done criteria
- [ ] Users can estimate cost both from a local file/directory and from a hypothetical size + duration before any deposit or upload