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.

components/settings/about-data-settings.tsx

@@ -11,6 +11,7 @@ import { useUpdateStore } from '@/stores/update-store';
 import { ExternalLink } from 'lucide-react';
 import { cn } from '@/lib/utils';
 import { getPathPrefix } from '@/lib/browser-navigation';
+import { clearCachedData } from '@/lib/clear-cached-data';
 import { SpamSiegeGame } from './spam-siege-game';
 
 const APP_VERSION = process.env.NEXT_PUBLIC_APP_VERSION || "0.0.0";
@@ -54,6 +55,7 @@ export function AboutDataSettings() {
     useSettingsStore();
   const { settingsSyncEnabled } = useConfig();
   const [showResetConfirm, setShowResetConfirm] = useState(false);
+  const [showRefreshConfirm, setShowRefreshConfirm] = useState(false);
   const fileInputRef = useRef<HTMLInputElement>(null);
   const { isFeatureEnabled } = usePolicyStore();
   const [showGame, setShowGame] = useState(false);
@@ -105,6 +107,15 @@ export function AboutDataSettings() {
     reader.readAsText(file);
   };
 
+  const handleRefreshCache = () => {
+    if (showRefreshConfirm) {
+      clearCachedData(); // reloads the page
+    } else {
+      setShowRefreshConfirm(true);
+      setTimeout(() => setShowRefreshConfirm(false), 5000);
+    }
+  };
+
   const handleReset = () => {
     if (showResetConfirm) {
       resetToDefaults();
@@ -187,6 +198,16 @@ export function AboutDataSettings() {
         </SettingItem>
         )}
 
+        <SettingItem label={t('refresh_cache.label')} description={t('refresh_cache.description')}>
+          <Button
+            variant={showRefreshConfirm ? 'default' : 'outline'}
+            size="sm"
+            onClick={handleRefreshCache}
+          >
+            {showRefreshConfirm ? tCommon('yes') : t('refresh_cache.button')}
+          </Button>
+        </SettingItem>
+
         <SettingItem label={t('reset_settings.label')} description={t('reset_settings.description')}>
           <Button
             variant={showResetConfirm ? 'destructive' : 'outline'}