Settings: View preferences (`viewingPreference`) and link previews (`showPreviews`)

[Adron]

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

• viewingPreference — All / My / Following / Followers, consumed by the feed (see the
  composer & feed epic; this issue owns the Settings half).
• showPreviews — whether link-preview cards render at all. Today the app always renders them.

Tests: PATCH round-trip; previews suppressed when off.

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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 #93 (branch issue/33-settings-view-preferences). Closing here — review and any follow-up happens on the PR.

Two facts this settled for the rest of the epic:

viewingPreference wire values are my_messages, all_messages, followers_only, following_only — confirmed live (the server returns the allow-list in its 400). The first implementation pass guessed all/mine/following/followers, which would have been rejected on every write; the tests now pin the real literals.

All fifteen PATCH /api/user/update fields are modelled, nullable and omitted-when-null, with a reusable SettingsGroup/SettingsRadioRow/SettingsSwitchRow kit — so #32, #34, #35, #36 and #37 are now UI-only work on top of this.

One decision left open for #19: :feature:messages cannot read SettingsRepository without either implementation(project(":feature:profile")) (no feature module depends on another today) or promoting the repository to a shared module.