feat(settings): add "Refresh cached data" recovery action

When the mailbox view gets into a stale or wrong state, the only escape
was the browser's "clear site data" — which also wipes the saved account
list, forcing a re-login of every account.

Add a non-destructive "Refresh cached data" button under Settings →
Data. It clears the server-derived caches (contacts, calendars,
identities, per-account snapshots) and reloads so they re-fetch fresh,
while preserving accounts, sessions, settings, themes and user content
(templates, S/MIME). Two-click confirm to avoid an accidental reload.

English strings added across all locales (translation follow-up); unit
tests cover the cache-clear (keeps account-registry/auth/prefs) and the
reload.

lib/__tests__/clear-cached-data.test.ts

@@ -0,0 +1,50 @@
+import { describe, it, expect, beforeEach, vi, afterEach } from 'vitest';
+import { clearCachedData } from '../clear-cached-data';
+
+describe('clearCachedData', () => {
+  let reload: ReturnType<typeof vi.fn>;
+  let originalLocation: Location;
+
+  beforeEach(() => {
+    localStorage.clear();
+    reload = vi.fn();
+    originalLocation = window.location;
+    Object.defineProperty(window, 'location', {
+      configurable: true,
+      value: { ...originalLocation, reload },
+    });
+  });
+
+  afterEach(() => {
+    Object.defineProperty(window, 'location', { configurable: true, value: originalLocation });
+  });
+
+  it('clears re-fetchable caches but keeps accounts, sessions and prefs', () => {
+    localStorage.setItem('contact-storage', '1');
+    localStorage.setItem('calendar-storage', '1');
+    localStorage.setItem('identity-storage', '1');
+    localStorage.setItem('calendar-notification-storage', '1');
+    // Must survive — losing these is exactly the pain we're avoiding.
+    localStorage.setItem('account-registry', 'accounts');
+    localStorage.setItem('auth-storage', 'session');
+    localStorage.setItem('settings-storage', 'prefs');
+    localStorage.setItem('template-storage', 'my templates');
+
+    clearCachedData();
+
+    expect(localStorage.getItem('contact-storage')).toBeNull();
+    expect(localStorage.getItem('calendar-storage')).toBeNull();
+    expect(localStorage.getItem('identity-storage')).toBeNull();
+    expect(localStorage.getItem('calendar-notification-storage')).toBeNull();
+
+    expect(localStorage.getItem('account-registry')).toBe('accounts');
+    expect(localStorage.getItem('auth-storage')).toBe('session');
+    expect(localStorage.getItem('settings-storage')).toBe('prefs');
+    expect(localStorage.getItem('template-storage')).toBe('my templates');
+  });
+
+  it('reloads so data is re-fetched fresh', () => {
+    clearCachedData();
+    expect(reload).toHaveBeenCalledOnce();
+  });
+});