fix(identity): sync default sender identity per account (#507)

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.

components/identity/identity-manager-modal.tsx

@@ -9,6 +9,7 @@ import { ConfirmDialog } from '@/components/ui/confirm-dialog';
 import { IdentityForm } from './identity-form';
 import { useIdentityStore } from '@/stores/identity-store';
 import { useAuthStore } from '@/stores/auth-store';
+import { useAccountStore } from '@/stores/account-store';
 import { useSettingsStore } from '@/stores/settings-store';
 
 function useSyncIdentities() {
@@ -207,15 +208,16 @@ export function IdentityManagerModal({ isOpen, onClose }: IdentityManagerModalPr
 
   const handleSetPrimary = useCallback((identity: Identity) => {
     setPreferredPrimary(identity.id);
-    // Persist to the synced settings (keyed by username, matching how
-    // loadIdentities reads it back) so the choice survives a new browser /
-    // cleared site data and reaches other devices (#507).
-    const username = useAuthStore.getState().username || '';
-    if (username) {
+    // Persist the choice per account in the synced settings store so it
+    // survives clearing site data, follows the user across devices, and shows
+    // up in exported settings (issue #507). JMAP identity ids are account-
+    // scoped, so the default is keyed by the active account.
+    const activeAccountId = useAccountStore.getState().activeAccountId;
+    if (activeAccountId) {
       const current = useSettingsStore.getState().preferredIdentityIds;
       useSettingsStore.getState().updateSetting('preferredIdentityIds', {
         ...current,
-        [username]: identity.id,
+        [activeAccountId]: identity.id,
       });
     }
     // Re-sort: move the preferred identity to the front