preflight upload check should account for minimum cost

[wjmelements]

--- Preflight Upload Check ---
Estimated costs:
  Per epoch (30s): 0.000000 USDFC
  Per day: 0.000000 USDFC
  Per month: 0.000000 USDFC
✓ Sufficient allowances available

Fixing this requires checking the current payment rate, perhaps from the payment rail.

[rvagg]

Let's call this a dupe of #308; also this is implemented in #375 but that PR needs some more thoughtful design work to avoid pilling on the jank.