Add account balance enquiry via GET /accounts/balance

The CBS API now exposes a balance endpoint, which it previously did not.
Several comments in the codebase asserted that no such endpoint existed
anywhere in the API; those are corrected here rather than left to mislead.

Balance is fetched lazily, per account, from the Account Details card:

- `accounts.balance` endpoint, `CbsAccountBalance` type, and a
  `getAccountBalance` query with `keepUnusedDataFor: 0` — a balance is the
  most perishable figure in the app and must not outlive its screen.
- The call is triggered by the customer, not by render. Every call reaches
  the core banking system, and a balance should not appear over someone's
  shoulder just for opening a screen.
- `supportsBalanceEnquiry` gates the control by account type only, with no
  status check: a dormant savings account cannot be debited but still has a
  balance. Deposits are excluded because the endpoint does not serve them.
- `ACCOUNT_BALANCE_NOT_AVAILABLE` added to the error catalogue as a
  non-retryable refusal, so the card states it instead of inviting a tap
  that gets the identical answer.

Also carries in-progress work already present in the tree: customer-facing
account type labels (`accountTypeLabel`) applied across the account card,
selector, mini statement, balance enquiry and MMID screens.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/services/api/cbsErrors.test.ts

@@ -9,9 +9,16 @@ import {
 } from './cbsErrors';
 
 describe('error code catalogue', () => {
-  it('covers all 45 documented codes, with no duplicates', () => {
-    expect(CBS_ERROR_CODES).toHaveLength(45);
-    expect(new Set(CBS_ERROR_CODES).size).toBe(45);
+  it('covers all 46 documented codes, with no duplicates', () => {
+    expect(CBS_ERROR_CODES).toHaveLength(46);
+    expect(new Set(CBS_ERROR_CODES).size).toBe(46);
+  });
+
+  it('does not invite a retry when the bank withholds a balance', () => {
+    // The account exists and the bank simply will not price it. Repeating the
+    // call gets the same refusal, so the card must not offer a retry.
+    expect(isCbsErrorCode('ACCOUNT_BALANCE_NOT_AVAILABLE')).toBe(true);
+    expect(isRetryableError('ACCOUNT_BALANCE_NOT_AVAILABLE')).toBe(false);
   });
 
   it('knows the cheque book codes', () => {