fix(ui): replace f64 round-trip with integer arithmetic in max amount calculations

[thepastaclaw]

Issue

The "Max" button in transfer and withdraw screens uses a floating-point round-trip (u64→f64→u64) to subtract fee estimates from the available balance. This can cause precision loss with large credit amounts, potentially resulting in incorrect max values.

Changes

Replace the floating-point arithmetic with integer saturating_sub:

// Before (precision loss possible)
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;

// After (exact integer arithmetic)
let max_amount_credits = self.max_amount.saturating_sub(20_000_000);

• transfer_screen.rs: 0.0002 DASH = 20,000,000 credits
• withdraw_screen.rs: 0.005 DASH = 500,000,000 credits

saturating_sub also handles the edge case where balance < fee (returns 0 instead of underflowing).

Validation

• cargo check ✅
• cargo fmt ✅

Summary by CodeRabbit

Release Notes

• Refactor
  • Optimized internal calculation logic for transfer and withdrawal amount limits to improve performance and reliability.

[coderabbitai]

Caution

Review failed

The pull request is closed.

📝 Walkthrough

Walkthrough

Two UI screen components replace floating-point arithmetic calculations with saturating integer subtraction to determine maximum transferable and withdrawable amounts after fee deductions. This eliminates float-based operations while preserving external behavior.

Changes

Cohort / File(s)	Summary
Transfer/Withdraw Max Amount Calculation src/ui/identities/transfer_screen.rs, src/ui/identities/withdraw_screen.rs	Replaced floating-point fee calculation with direct saturating subtraction. Transfer screen subtracts 20,000,000 credits (0.0002 DASH); withdraw screen subtracts 500,000,000 credits (0.005 DASH). Eliminates float operations and simplifies fee handling logic.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~4 minutes

Poem

🐰 No floats to fret, just subtraction clean,
Saturating sums, the sharpest I've seen!
Twenty millions, five hundred more,
Integer math is what we adore! ✨

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• Create PR with unit tests
• Post copyable unit tests in a comment

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[thepastaclaw]

Duplicate of #622 — sub-agent created the same fix independently. Closing in favor of #622.

[thepastaclaw]

@coderabbitai review

[coderabbitai]

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