chore(contract): sync ocp protos to e3d25a85

[bmc08gt]

Syncs proto/ to ocp-protobuf-api@e3d25a85 and adds the 0.6.0 changelog entry. Upstream #67 is docs only; #68 is the one contract change.

balance/v1/ocp_balance_service.proto, additive only:

• GetBalancesRequest.currency_codes (field 3): lowercase ISO 4217 codes. Empty keeps today's core-mint-only response.
• OwnerBalance.fiat_values_by_currency (field 4) and MintBalance.fiat_values_by_currency (field 3): map<string, double>, populated only for the requested codes.

Nothing renumbered or removed. Neither app sends currency_codes yet, so both upgrade with no behaviour change.

Release as 0.6.0 after merge. App PRs pinning it (drafts until it's published):

• feat(chat): handle the E2EE contract additions and bump the contract packages code-android-app#1573
• feat(chat): handle the E2EE contract additions and bump the contract packages code-ios-app#870

The flipcash2 sync ships alongside: code-payments/flipcash2-client-protocol#18