The Android and iOS gomobile bindings carried two near-identical copies of
the profile manager. Move the shared implementation into a new client/mobile
package and reduce both bindings to thin adapters that only translate to
gomobile-friendly types (gomobile binds per package, so the Profile /
ProfileArray wrappers have to stay platform-side).
Also bring the account-email layer over to the shared package: an SSO login
records the account under <stem>.account.json so the next login can pass it
as an OIDC login_hint. Logout keeps it, profile removal drops it. The suffix
deliberately differs from .state.json, which the engine's state manager owns
in the same directory on mobile.
Adds profilemanager.Prefs (namespaced per-profile preference store) and its
cleanup in ServiceManager.RemoveProfile, exposed through the shared manager
as ProfilePrefs.
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