Phase 3: Functional fixes — amount validation, BigDecimal money, checkUserExists

[devin-ai-integration]

Summary

Phase 3 of the modernization plan: version-independent functional bug fixes. Stacked on the Phase 2 PR (#48) (which is itself on Phase 1). Backend suite grows 21 → 35 tests, all green.

1. checkUserExists email bug

-if (checkUsernameExists(username) || checkEmailExists(username)) {
+if (checkUsernameExists(username) || checkEmailExists(email)) {

Previously a signup with a new username but an already-registered email slipped through (it checked the username against the email index twice).

2. Money handling → BigDecimal

PrimaryTransaction.amount / SavingsTransaction.amount changed from double to BigDecimal, and AccountService.deposit/withdraw now take BigDecimal instead of double. This removes binary floating-point drift on currency and the Double.parseDouble(...) calls in the service/controller layers.

3. Amount validation + sufficient funds (no unhandled NumberFormatException)

New AmountUtil centralizes parsing/validation, plus an InsufficientFundsException:

BigDecimal parsePositiveAmount(String)   // throws IllegalArgumentException on null/non-numeric/<= 0
void requirePositive(BigDecimal)
void requireSufficientFunds(BigDecimal balance, BigDecimal amount) // throws InsufficientFundsException

• AccountController deposit/withdraw parse via AmountUtil and, on bad input, redirect to …?error instead of throwing a raw NumberFormatException (500).
• AccountServiceImpl / TransactionServiceImpl validate positivity and check funds before subtracting, for deposits, withdrawals, between-account transfers, and transfers to a recipient.

4. Consistent transfer category labels

Between-accounts transfers were inconsistent: Primary→Savings recorded type "Account" while Savings→Primary recorded "Transfer". Both now use "Account".

Tests (+14)

• AmountUtilTest: valid/negative/zero/non-numeric/null parsing, over-draw vs exact-balance funds check.
• AccountServiceIntegrationTest: non-positive deposit, zero withdraw, insufficient-funds withdraw.
• TransactionServiceIntegrationTest: negative / non-numeric / insufficient-funds between-accounts transfer.
• UserServiceIntegrationTest: checkUserExists detects duplicate email under a new username.

Verified: mvn clean test → 35 tests, 0 failures.

Note: TransferController's recipient/toSomeoneElse endpoints now surface IllegalArgumentException/InsufficientFundsException on bad input; user-facing error handling there is folded into the Phase 4 controller work (state-changing GET→POST/DELETE + ownership checks).

Link to Devin session: https://app.devin.ai/sessions/82be2e41fda54343a20a9b73e01c0631
Requested by: @gaelkekatos-jpg

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