feat: add pre-upload cost estimation

[BravoNatalie]

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

[SgtPooki]

2026-06-16

I'd avoid a new top-level estimate command and the explicit --bytes/--months flags. Two existing commands already cover both cases:

1. File estimate -> filecoin-pin add <path> --dry-run. No --dry-run today, but the path case is exactly what it'd do: pack the CAR, compute the packed size, print cost + copies, and stop before any upload or deposit. Reuses the add-flow code that already sizes a CAR.

2. "How much for X GiB for Y months?" -> a preview/dry-run of payments fund (or payments setup) that accepts size + duration and derives the suggested deposit. This is the same thing DoD bullet 3 already asks for, and matches the "like fund but a preview version" framing from the UX notes. No standalone hypothetical estimator needed.

So the scope collapses to: add --dry-run to add, and let the funding flow take a size/duration target to preview the deposit. The --bytes/--months standalone estimate can drop.

Done criteria should be rescoped around those two changes.