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

The default sender identity (`preferredPrimaryId`) lived only in the
browser-local `identity-storage` Zustand store and was never written to
the server-side synced settings. As a result the choice was lost when
clearing site data or switching browsers, and never appeared in the
exported settings JSON.

Persist the default identity 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.

- settings-store: add `preferredIdentityIds` to state, defaults, export
  (so it shows in exported JSON), import (with a non-record guard),
  rehydrate coercion, and a v6->v7 migration.
- auth-store: add `applyPreferredIdentity()`, invoked in every
  `loadFromServer().finally()` (login / OAuth / SSO / switch / restore) so
  the synced default reorders the active account's identities once server
  settings load (the composer defaults From to identities[0]).
- identity-manager-modal: the star action also writes the choice to the
  synced per-account map, triggering server sync + export inclusion.
- identity-store: keep `preferredPrimaryId` in local persist as a sync-off
  fallback; synced settings are the durable cross-device source of truth.
- tests: per-account independence, export/import round-trip, non-record
  import guard, and v6->v7 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