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();
+  });
+});

lib/clear-cached-data.ts

@@ -0,0 +1,45 @@
+import { evictAll } from '@/lib/account-state-manager';
+
+// localStorage keys holding server-derived, re-fetchable caches. The "Refresh
+// cached data" action clears these so a stale or wrong-account view can be
+// fixed WITHOUT signing out (which would drop the whole account list).
+//
+// Deliberately excluded:
+//  - 'account-registry' / 'auth-storage'  → keep accounts + sessions
+//  - 'settings-storage' / 'theme-storage' / 'locale-storage' → user prefs
+//  - 'template-storage' / 'smime-preferences' → user-created content
+const CACHE_STORAGE_KEYS = [
+  'identity-storage',
+  'contact-storage',
+  'calendar-storage',
+  'calendar-notification-storage',
+];
+
+/**
+ * Clear cached, server-derived data (contacts, calendars, identities, and the
+ * in-memory per-account snapshots), then reload so everything is re-fetched
+ * fresh for the active account. Accounts and sessions are preserved — this is
+ * the non-destructive alternative to the browser's "clear site data", which
+ * also wipes the account list.
+ */
+export function clearCachedData(): void {
+  // Drop the in-memory per-account store snapshots so a reload can't restore
+  // stale cached state for any account.
+  try {
+    evictAll();
+  } catch {
+    /* snapshots are best-effort */
+  }
+
+  if (typeof window === 'undefined') return;
+
+  for (const key of CACHE_STORAGE_KEYS) {
+    try {
+      window.localStorage.removeItem(key);
+    } catch {
+      /* ignore storage access errors */
+    }
+  }
+
+  window.location.reload();
+}