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/endpoints.ts

@@ -29,6 +29,16 @@ export const endpoints = {
      */
     list: '/accounts',
     mmid: '/accounts/mmid',
+    /**
+     * One transactable account's current balance, by `accountNumber` query
+     * parameter. Read live from the bank on every call — nothing is cached
+     * here, and nothing about a balance may be.
+     *
+     * SAVINGS and CURRENT only. A deposit's balance is a term-deposit concept
+     * and lives on `depositDetail` below, which is why the bank keeps the two
+     * apart rather than answering both from one endpoint.
+     */
+    balance: '/accounts/balance',
     /**
      * A page of ledger entries for one account. Takes the full account number
      * from `list` above, and an optional `fromDate`/`toDate` pair that the