Use exact CAP balances for max stake actions

[qingfeng312]

Fixes #60.

Summary

• keep the displayed CAP balance rounded for readability
• use the exact wallet balance when the Stake CAP max row is clicked
• use the exact staked CAP amount when the Unstake CAP max row is clicked

This prevents the max shortcut from writing a rounded display value that can exceed the actual spendable/staked amount.

Validation

• git diff --check
• Not run: full build dependencies are not installed in this clean checkout.

Payout address: 0xe87b4889baeee4ed60a1b2bfc7b3a6a17bce4ad6