* Removed månatlig skatt from onboarding flow
* fix: convert DB null to undefined for moms_period in onboarding step 3
The moms_period field used a type cast (`as MomsPeriod | undefined`) instead
of null coalescing (`?? undefined`), causing Zod validation to reject the
DB-default null value since `.optional()` only accepts undefined.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
* feat: add VAT accounting step to onboarding process
* feat: replace accounting method select with checkboxes for better UX
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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
