Closes #3210

Root cause: Preview path incorrectly squared the balance when deriving base amount; volume units (percentage/USD) weren’t applied; no clamping to spendable.

Resolution:

• Compute base amount from TradeVolume:
• Percentage: spendable * percentage (fraction 0–1).
• USD: usdVolume / baseUsdPrice, then clamp to spendable.
• Use live order available amounts when present.
• Derive rel amount from USD price ratio with margin.

Scope: UI preview for makerbot Orders list only; no backend/API changes.

Confirmation test

• Percentage case:
  • Set makerbot max volume to 10% and create an order; open Orders tab.
  • Expected “Send” = spendable(base) * 0.10, never > spendable.
• USD case:
  • Set makerbot volume in USD (e.g., $100).
  • Expected “Send” ≈ $100 / baseUsdPrice, clamped to spendable.
• Confirm existing orders use availableAmount and do not exceed balance.

Files touched

• lib/bloc/market_maker_bot/market_maker_order_list/market_maker_bot_order_list_repository.dart

Checklist

• Correct preview amounts for percentage-volume.
• Correct preview amounts for USD-volume.
• Values never exceed spendable balance.
• Uses live order available amounts when present.
• No new lints in modified file.