Adds the 'VAT registration & reclaim' section to Settings → Accounting: a
'VAT-registered' toggle (charge output + reclaim input VAT) and a multi-select
of countries whose input VAT is reclaimable (default domestic CH/LI). Wires
accounting.service + the backend keys added earlier (accounting_vat_registered,
accounting_vat_reclaim_countries). i18n en/de. The report VAT-payable math that
consumes these is the next slice.