Expose storage price calculations and meaningful stamp purchase errors

[crtahlin]

Summary

Users currently need to call Gnosis RPC and inspect smart contracts to calculate storage costs. The Bee node has this information (via /chainstate) but doesn't expose it in a user-friendly way with calculations.

Additionally, when stamp purchases fail (e.g., duration too short), users get unhelpful error messages.

Current Behavior

• /chainstate returns raw currentPrice (PLUR per chunk per block), but no calculations
• Stamp purchase failures return generic errors without explaining why or what's needed

Expected Behavior

1. Storage price calculations endpoint

Extend /chainstate or add new endpoint that provides:

• Current storage price (PLUR per chunk per block) - already in chainstate
• Calculator: given amount + depth → expected duration
• Calculator: given duration + depth → required amount
• Calculator: given duration + depth → total cost to pay

2. Meaningful stamp purchase errors

When a stamp purchase fails due to constraints, return a clear error explaining:

• Why it failed (e.g., "duration less than minimum 24 hours")
• What's required (e.g., "use amount of at least XXX for 24h minimum")
• Include current price context so user can adjust

Use Case

Developers and users want to purchase stamps without needing to:

1. Query Gnosis RPC directly
2. Understand PLUR/chunk/block calculations manually
3. Guess why their purchase failed

This would significantly improve the stamp purchasing UX.