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.

stores/__tests__/settings-store-preferred-identity.test.ts

@@ -0,0 +1,58 @@
+import { describe, it, expect, beforeEach } from 'vitest';
+import { useSettingsStore } from '../settings-store';
+
+describe('settings-store per-account preferredIdentityIds (issue #507)', () => {
+  beforeEach(() => {
+    useSettingsStore.setState({ preferredIdentityIds: {} });
+  });
+
+  it('defaults to an empty record (no account has a synced default)', () => {
+    expect(useSettingsStore.getState().preferredIdentityIds).toEqual({});
+  });
+
+  it('keeps each account default independent', () => {
+    useSettingsStore.setState({
+      preferredIdentityIds: { 'acct-1': 'b', 'acct-2': 'c' },
+    });
+    const map = useSettingsStore.getState().preferredIdentityIds;
+    expect(map['acct-1']).toBe('b');
+    expect(map['acct-2']).toBe('c');
+    expect(map['acct-3']).toBeUndefined();
+  });
+
+  it('round-trips through export -> import so the choice survives clearing site data', () => {
+    useSettingsStore.setState({ preferredIdentityIds: { 'acct-1': 'b' } });
+    const json = useSettingsStore.getState().exportSettings();
+    // Appears in exported JSON (issue #507 acceptance criterion).
+    expect(JSON.parse(json).preferredIdentityIds).toEqual({ 'acct-1': 'b' });
+
+    // Simulate a fresh browser: clear, then import the exported settings.
+    useSettingsStore.setState({ preferredIdentityIds: {} });
+    expect(useSettingsStore.getState().importSettings(json)).toBe(true);
+    expect(useSettingsStore.getState().preferredIdentityIds).toEqual({ 'acct-1': 'b' });
+  });
+
+  describe('importSettings non-record guard', () => {
+    it('ignores a legacy array shape', () => {
+      useSettingsStore.setState({ preferredIdentityIds: { 'acct-1': 'b' } });
+      const ok = useSettingsStore.getState().importSettings(
+        JSON.stringify({ preferredIdentityIds: ['b'] }),
+      );
+      expect(ok).toBe(true);
+      expect(useSettingsStore.getState().preferredIdentityIds).toEqual({ 'acct-1': 'b' });
+    });
+
+    it('ignores a null value', () => {
+      useSettingsStore.setState({ preferredIdentityIds: { 'acct-1': 'b' } });
+      useSettingsStore.getState().importSettings(JSON.stringify({ preferredIdentityIds: null }));
+      expect(useSettingsStore.getState().preferredIdentityIds).toEqual({ 'acct-1': 'b' });
+    });
+
+    it('accepts a proper per-account record', () => {
+      useSettingsStore.getState().importSettings(
+        JSON.stringify({ preferredIdentityIds: { 'acct-9': 'a' } }),
+      );
+      expect(useSettingsStore.getState().preferredIdentityIds).toEqual({ 'acct-9': 'a' });
+    });
+  });
+});