The default sender identity (`preferredPrimaryId`) lived only in the
browser-local `identity-storage` store and was never written to the synced
settings, so the choice was lost on clearing site data / switching browsers and
never appeared in exported settings.
Persist it in the synced settings store, keyed **per account**
(`preferredIdentityIds: Record<accountId, identityId>`), mirroring the existing
per-account `allMailFolderIds`. Per-account keying is required because JMAP
identity ids are account-scoped and would otherwise collide across accounts /
the unified mailbox.
This supersedes the earlier username-keyed fix that had landed on main: the
username-keyed map, `loadIdentities()` fallback write, and the
`applyPreferredIdentityOrdering` store action (plus its settings-store hook)
are removed so a single account-keyed mechanism remains.
- settings-store: `preferredIdentityIds` (accountId -> identityId) in state,
defaults, export, import (non-record guard), rehydrate coercion, v6 migration.
- auth-store: `applyPreferredIdentity(accountId?)` reorders the active account's
identities once synced settings load, and performs the one-time migration of
the pre-#507 browser-local default into the synced map (keyed by accountId).
Invoked from every `loadFromServer().finally()` (login / OAuth / SSO / switch
/ restore). `loadIdentities()` now only applies the local fallback ordering.
- identity-manager-modal: the star action writes the choice by `activeAccountId`.
- identity-store: `preferredPrimaryId` kept as a local (sync-off) fallback.
- tests: per-account independence, export/import round-trip, import guard, and
applyPreferredIdentity reorder / active-account gating / local-default
migration.
