refactor(sdk): move subaccounts() to PrivateTransfersBuilder (#905)

`subaccounts(dappName)` hung on `PrivateTransfersInterface`, but a sub-account
`invoke(nonce, {calls})` produces builder operations — so it belongs in the
builder namespace. Moving it there also fixes a latent bug: the old
`SubAccountsBuilderImpl.invoke` spun up its own `transfers.build()` internally,
discarding any `ExecuteOptions` the caller passed. Now `subaccounts` is reached
via `transfers.build(options).subaccounts(dappName)` and its `invoke` chains onto
that same builder, so options/context flow through.

`build()` threads the sub-account deps (anonymizer address + viewing-key getter)
into the builder; the anonymizer-address requirement now throws from
`builder.subaccounts(...)`. Reads (`partialCommitment`/`commitment`) are unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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