F-201: GET /pos/reports/cash-close/:id with financial summary, sales
by state, payments breakdown, items sold. Extended /pos/sessions/:id.
F-202: Cash close email sent on session close to smtpReportEmail
(best-effort). smtpReportEmail field added to admin SMTP settings.
F-203: Admin POS terminal config: selfpayMode, closeSessionRequiresPin,
closeSessionPin (4-6 digits) with dedicated settings section.
