Settings: Message settings group (default visibility, max length, messages per page, advanced post settings)

[Adron]

Part of #31 — Settings & preferences parity — most of PATCH /api/user/update is unreachable from Android

Adds the four message-behaviour preferences and makes the composer and feed honour them.

Field	Effect
defaultPubliclyVisible	The composer's starting visibility
maxMessageLength	The account's character limit (default 666)
messagesPerPage	Feed page size
showAdvancedPostSettings	Whether the composer's advanced (gear) options are shown

Note the web places the character limit under Profile, not Message Settings — mirror the
grouping users will have learned there, or make the search/label obvious either way.

Tests: PATCH carries each field; composer/feed read them.

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Filed from the parity review of feature/parity-wip @ 14d7795 against interlinedlist.com (OpenAPI /api/openapi.json, 233 paths) and the public help centre.

[Adron]

Implemented in #99 (branch issue/32-settings-message-group). Closing here — review and any follow-up happens on the PR.

The grouping question in the issue is resolved from the docs rather than guessed: /help/messages says outright "Each user has a configurable message length limit (default 666 characters). Adjust it in Settings, then Profile (not Message Settings)." So the character limit went under a new Profile group, messages-per-page under View preferences (where /help/settings puts it, documented range 10–30), and only default visibility + advanced post settings under Message settings.

Also settled: the request-side type question #33 left open. The generated spec types these fields as string but types isPrivateAccount as boolean — an inconsistency that only makes sense as a generator artifact from a coercing handler, and the spec's own 200 example plus the live GET /api/user both return maxMessageLength: 666 as a number. Natural JSON types it is, now asserted in tests.

Deliberately not done here: the composer and feed do not yet consume these values. That is the :feature:messages lane — #24 owns the character limit and #19 the feed preferences (#18 already consumes defaultPubliclyVisible).