chore(contract): sync flipcash2 protos to 993127b

[bmc08gt]

Syncs flipcash2 protos from d082b0a3 to 993127b5 (flipcash2-protobuf-api #130 and #131) and adds the 0.15.0 changelog entry. Only chat.v1 moved, and every change is an addition:

• New RPC Chat.GetMentionSuggestions. It returns a ranked pool of up to 200 people to suggest after an @ in a group chat. The pool isn't paged, and the client filters it locally. A DM returns DENIED.
• GetMentionSuggestionsRequest (chat_id, auth) and GetMentionSuggestionsResponse (result: OK, DENIED, NOT_FOUND; repeated suggestions).
• MentionSuggestion: a UserProfile plus an optional last_sent_at. It is not a Member.

No existing field or enum case is renamed or renumbered.

The changelog commit also regenerates ContractInfo.swift, whose version string follows the top changelog heading (0.15.0-dev).

Both apps build against this branch through the local overrides. Their pin bumps are draft PRs that wait on the 0.15.0 release:

• Android: feat(chat): suggest @mentions from the server's pool code-android-app#1647
• iOS: feat(chat): suggest @mentions from the server's pool code-ios-app#942

After merge, run publish.yml at 0.15.0.