Consolidate the two writers of account preferences behind one owner

[Adron]

Follow-up from #33 and #19, both of which needed the same account preference and deliberately did
not create a feature→feature dependency to get it.

Current state

Two independent paths now read and write PATCH /api/user/update:

1. :feature:profile → SettingsRepository (from Settings: View preferences (viewingPreference) and link previews (showPreviews) #33) — models all fifteen fields, backs the
   Settings screen, @Singleton with a process-scoped in-memory cache.
2. :core:network → ViewingPreferenceStore (from Feed: view preferences (All / My / Following / Followers) and onlyMine #19) — a narrow accessor for
   viewingPreference so :feature:messages can drive the feed switcher.

Each has its own in-flight state and neither knows about the other's writes.

User-visible symptom: change the view preference in Settings, and an already-composed feed
switcher still shows the old value until the feed screen is re-entered.

Why it happened

No feature module in this repo depends on another — that is a deliberate convention that keeps
modules conflict-free for parallel work. :feature:messages therefore could not read
:feature:profile's repository, and duplicating a narrow accessor in :core:network was the
smaller evil at the time.

What to do

Promote a single account-preferences owner — a :core:preferences (or similar) module holding the
UserSettings model, the partial-PATCH call and one observable cache — and have both
:feature:profile's Settings screen and :feature:messages's feed switcher consume it. Then delete
ViewingPreferenceStore and fold SettingsRepository onto the shared owner.

Best done once the Settings epic (#31) has finished landing, so it is not fighting five parallel
branches.