fix: better error logging and prevent transactions over total balance

[sqhell]

● Changes

• Added client-side balance validation to Transfer flow - Validates amount against spendable balance with 0.0001 ZEC fee buffer before allowing users to proceed past input screen
• Added fee buffer validation to Shield flow - Prevents shielding when transparent balance is below 0.0015 ZEC minimum (includes 0.0005 ZEC fee buffer)
• Enhanced error handling for InsufficientFunds errors - Parses Rust WASM errors to extract and display exact available/required amounts with clear, actionable messages
• Improved error logging - Added comprehensive error logging including full stack traces, error types, and parsed amount breakdowns for debugging

Tests

Build and run the dev server

cd /home/skynet/ztest/WebZjs
just build
yarn dev

Test 1: Transfer validation

Navigate to Transfer/Send screen

Try to send amount > available balance

Expected: Error shown immediately, cannot proceed

Error message shows: "Insufficient balance. Available (after fees): X.XXXXXXXX ZEC"

Test 2: Shield validation

Navigate to Shield Balance screen with < 0.0015 ZEC transparent balance

Expected: Shield button disabled

Error shows: "Minimum balance required: 0.0015 ZEC (includes transaction fees)"

Test 3: Enhanced error messages (if validation bypassed)

Check browser console for detailed error breakdown:

- Full error object logged

- Parsed amounts: Available, Required, Shortfall in both zatoshis and ZEC

- User sees elegant message with exact amounts needed

Test 4: Valid transactions

Try transfer with amount < (balance - fee buffer)

Try shield with balance > 0.0015 ZEC

Expected: Both proceed normally through confirmation and execution

Issues

• Fixes user-reported issue: "Insufficient Funds" errors showing ugly WASM error messages instead of user-friendly validation
• Prevents users from reaching confirmation screen with insufficient balance
• Addresses lack of fee consideration in UI validation logic