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/types/cbs.ts

@@ -251,6 +251,19 @@ export interface CbsAccount {
   mmid: string | null;
 }
 
+/**
+ * One account's balance, as `GET /accounts/balance` returns it.
+ *
+ * Deliberately not folded into `CbsAccount`: `GET /accounts` does not carry a
+ * balance, and the two are fetched at different moments for different reasons.
+ * Keeping them apart is what stops a stale figure riding along on an account
+ * list that was cached for some other purpose.
+ */
+export interface CbsAccountBalance {
+  /** In rupees. The bank's live figure at the moment of the call. */
+  balance: number;
+}
+
 /**
  * One deposit as `GET /accounts/deposits/detail` returns it.
  *