chore(contract): sync flipcash2 protos to ec66e6b

[bmc08gt]

Syncs flipcash2 protos from 993127b5 to ec66e6b1 (flipcash2-protobuf-api #132 and #133) and adds the 0.16.0 changelog entry. chat.v1, event.v1, messaging.v1 and profile.v1 moved; blob.v1 changed only in comments.

One source break: StartChatRequest's group case is now public_group, and GroupChatParameters is now PublicGroupChatParameters. The field number is still 1, so it's wire-compatible, but both apps stop compiling until they rename their call sites.

Added:

• Private groups: a private_group case on StartChatRequest (PrivateGroupChatParameters: title, optional picture).
• Seven Chat RPCs for the lobby and keys: EnterLobby, LeaveLobby, GetLobbyMembers, AdmitLobbyMember, DenyLobbyMember, SetKeyEnvelope, GetKeyEnvelope.
• KeyEnvelope, Lobby, LobbyMember, LobbyUpdate, LobbyUpdateBatch, and ChatUpdate.lobby_updates (= 9).
• Metadata.is_private (= 14) and in_lobby (= 15).
• EncryptedContent.Scheme.CHAT_KEY_XCHACHA20POLY1305 (= 2), and ENCRYPTION_REQUIRED on SendMessageResponse.Result (= 3) and EditMessageResponse.Result (= 6).
• UserProfile.bio (= 12) and cover_picture (= 13), set through new Profile.SetBio and Profile.SetCoverPicture RPCs.

No field or enum case is renumbered; every new enum case comes after the existing ones.

80d1052 regenerates ContractInfo.swift for the new changelog heading (0.16.0-dev).

Both apps build against this branch through the local overrides. Their pin bumps are draft PRs that wait on the 0.16.0 release:

• Android: feat(chat): adopt flipcash2 0.16.0 (private groups, bio, cover picture) code-android-app#1680
• iOS: feat(chat): adopt flipcash2 0.16.0 (private groups, bio, cover picture) code-ios-app#974

After merge, run publish.yml at 0.16.0.