feat: estimate confirmation time for a custom on-chain fee #1634

Problem or use case

On Bitkit Android 3.0.1 (195), after setting a custom on-chain fee, the confirm screen's "Confirming in" only says "Depends on the fee". In the case seen, the custom rate was 4 sat/vB while the network's fastest tier was 2 sat/vB, so the transaction was headed for the next block, but the screen gave no estimate.

Proposed solution

Estimate the confirmation time for a custom rate against the current fee tiers, the same way the preset speeds already do:

• At or above the fastest tier: "next block" (or the fast preset's time text).
• Between tiers: the time of the matching tier, for example "±10-20 minutes", with that tier's label below it.
• Below the slowest tier: keep a cautious message.

Notes

• Low priority.
• iOS not checked yet.