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.
