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.

locales/en/common.json

@@ -1637,6 +1637,11 @@
         "description": "View available keyboard shortcuts",
         "button": "View Shortcuts"
       },
+      "refresh_cache": {
+        "label": "Refresh cached data",
+        "description": "Reload contacts, calendars, and folders from the server. Keeps your accounts and sessions \u2014 fixes a stale or wrong view without signing out.",
+        "button": "Refresh"
+      },
       "reset_settings": {
         "label": "Reset Settings",
         "description": "Restore all settings to default values",