fix(ui): replace f64 precision-lossy max amount calculation with integer arithmetic

[thepastaclaw]

Replace floating-point round-trip (u64→f64→u64) with integer saturating_sub for fee reservation in Max button calculations on transfer and withdraw screens.

Problem

The original code converted credits to DASH as f64, subtracted a fee constant, then converted back to u64:

let max_amount_minus_fee = (self.max_amount as f64 / 100_000_000_000.0 - 0.0002).max(0.0);
let max_amount_credits = (max_amount_minus_fee * 100_000_000_000.0) as u64;

This loses precision in the floating-point round-trip. For example, a balance of exactly 20_000_001 credits would produce 0.00000000020000001 in f64, subtract 0.0002, multiply back — and the result may differ from the mathematically correct 1 due to IEEE 754 representation.

Fix

Use integer arithmetic directly on the credit value:

// 0.0002 DASH = 20_000_000 credits
let max_amount_credits = self.max_amount.saturating_sub(20_000_000);

Cherry-picked from ralph/improvements (commit 1909415).

Validation

What was tested:

• cargo clippy --all-features --all-targets -- -D warnings — lint check
• cargo test --all-features --workspace — full workspace test suite

Results:

• All local commands passed
• Clippy CI check — pass (5m45s)
• Test Suite CI check — pass (8m27s)

Environment: Local macOS arm64; GitHub Actions CI (ubuntu-latest)

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[thepastaclaw]

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