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/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();
+}