test: buy + sell cubit subset, no PR-conflict overlap (+22 tests)

[TaprootFreak]

Summary

Stage 13 of the coverage push. Covers four cubits in the sell and buy flows whose source files are NOT touched by the currently-open #321 (dashboard buy actions, sell_page) or #332 (bitbox sign).

Cubit under test	Test file	Cases
`sell/cubits/sell_selected_bank_account/sell_selected_bank_account_cubit.dart`	`test/screens/sell/cubits/sell_selected_bank_account_cubit_test.dart`	3
`sell/cubits/sell_balance/sell_balance_cubit.dart`	`test/screens/sell/cubits/sell_balance_cubit_test.dart`	4
`sell/cubits/sell_bank_accounts/sell_bank_accounts_cubit.dart`	`test/screens/sell/cubits/sell_bank_accounts_cubit_test.dart`	6
`buy/cubits/buy_converter/buy_converter_cubit.dart`	`test/screens/buy/cubits/buy_converter_cubit_test.dart`	9

What each file covers

• sell_selected_bank_account_cubit: initial null, `selectBankAccount` emits the provided account, `selectBankAccount(null)` clears the selection.
• sell_balance_cubit: initial zero-balance derived from `AppStore.apiConfig.asset` + `primaryAddress`; subscribes to `BalanceRepository.watchBalance` on init; emits each pushed balance; `close()` cancels the subscription.
• sell_bank_accounts_cubit: Success with DTO → BankAccount mapping on init; LoadFailure on `getBankAccounts` throw; `add()` calls `createBankAccount` + re-fetches; `AddFailure` preserves prior accounts + carries error message; `deactivate()` calls `updateBankAccount(isActive=false)` + re-fetches; `UpdateFailure` preserves prior accounts.
• buy_converter_cubit: initial empty state; `onFiatChanged` debounces (100ms) and writes the converted shares; multiple rapid keystrokes only fire the LAST service call (pins the debounce contract); service error keeps state stable; `onSharesChanged` matches the output's fractional digits to the input (`'5'` → 2 digits, `'5.000'` → 3 digits — pins `_fractionDigits` behaviour); `onCurrencyChanged` re-fetches in the new currency and emits both fields; currency still flips on service error; `close()` cancels pending debounce timers so no service call happens after close.

Excluded (and why)

• `sell_payment_info`, `sell_confirm`, `sell_converter` (the parallel cubit to buy_converter), `buy_confirm`, `buy_payment_info` — each pulls in `real_unit_buy_payment_info_service` / `real_unit_sell_payment_info_service` / signing flows that are likely on PR fix: keep dashboard buy actions visible #321's path. Held back to avoid review conflicts.
• `sell_bitbox_cubit`, `hardware_connect_bitbox_cubit` — both directly touch BitBox; held while fix(bitbox): harden multi-page sign and KYC routing #332 is open.

Test plan

• `flutter analyze` on the four new files — clean
• `flutter test` — 22 / 22 passing locally
• CI green