Let the verifiedVatId useEffect handle the fetch instead of
calling fetchVAT directly in handleVerifyVatId. Prevents
VIES MS_MAX_CONCURRENT_REQ rate limiting.
Split isVatVerified into isVatChecked (user clicked Verify) and
isVatValid (VIES confirmed exempt). Button now shows Verify and
stays enabled after a failed check so the user can retry.
PlanSummary now has a Verify button for VAT ID instead of
auto-verifying on input. Shows company name and address from
VIES on successful verification, with warning on invalid IDs.
