[bmc08gt]

Client-side validation failures from Bugsnag group 6aba977 (ValidationException, 459 production events, 29 users) come from six different causes. The triage brief (.claude/plans/triage-6aba9772991833fd0964d3c3.md on docs/bugsnag-validation-triage) splits them by field. This PR covers the two client bugs it left open and the three user-input fields with no UI guard. username is fixed by the 0.24.0 pin already on code/cash, and message_ids by #1676.

Client bugs

• user_id: UserManager.clear() set accountId = NoId, an empty list, so the ?: return guards in AccountController and ProfileController passed it through. clear() now sets null, and the accountId getter reads an empty id as missing. Brief marks this cause as likely, from two Internal events.
• Short mint (addresses[0], address): I could not name the caller. The stack resumes from the main Handler. Two changes close the holes I could confirm: AppRouter.handleTokenLink took the raw path segment as a mint, so /token/<short> or a non-base58 segment now returns no deeplink, and TokenCoordinator.getTokenMetadata / getHistoricalMarketCapData now fail fast on a mint that is not 32 bytes (Mint.isValid) and trace the bad value and the method. If the persisted-selection or Room paths are the source, the next event will say so.

Input checks (limits from the pinned contracts: flipcash2-client-protocol 0.24.0, ocp-client-protocol 0.9.0)

• Create Group group.title: min_len 1, max_len 64 on PublicGroupChatParameters and PrivateGroupChatParameters. CreateGroupViewModel.hasTitle now uses ChatTitle, the rule Edit Group already uses (trimmed, code points). ChatTitle moved from :features:messenger to :shared:chat so :features:tipping can reach it.
• display_name outside onboarding: SetDisplayNameRequest, min_len 1, max_len 64. My Account and Tip Card setup now use isWithinDisplayNameLimit (code points, measured as typed because those sources send the text untrimmed). The Done-key guard in ConfirmNameChange now applies to every source, not only onboarding.
• Currency name: CheckAvailabilityRequest and LaunchRequest, max_len 32, pattern ^[!-~]([ -~]*[!-~])?$. New CurrencyName rule gates hasName (Next button) and the CheckName event, which the Done key reaches without the button gate.

All three checks follow the existing pattern of disabling the Next/Create button from state, as onboarding display name does. No strings, components or styles changed.

Tests: MintTest, UserManagerTest, AppRouterTest, ChatTitleTest, CreateGroupStateTest, NameEntryStateTest, CurrencyNameTest. Each limit has an at-limit and one-over case.