Fix/company creation bug (#212)

* feat: enhance JournalEntryForm with currency selection and exchange rate fetching

- Added currency selection to JournalEntryForm, allowing users to choose from multiple currencies (SEK, EUR, USD, GBP, NOK, DKK).
- Implemented fetching of exchange rates from Riksbanken API based on selected currency and entry date.
- Updated calculations for foreign amounts and SEK equivalents based on user input and fetched exchange rates.
- Improved form handling to reset currency-related fields when switching back to SEK.

feat: refactor WelcomeOnboarding to streamline company creation process

- Replaced direct company switching with a new server action to create a company from onboarding data.
- Added validation for fiscal period during onboarding steps, allowing for mid-month starts for the first fiscal period.
- Enhanced error handling and rollback mechanisms to ensure data integrity during company creation.

fix: update Step3TaxRegistration to allow flexible first-year start dates

- Modified date selection to include day, month, and year for the first-year start date.
- Updated validation messages to reflect changes in fiscal year start date handling.

test: expand validate-period-duration tests for fiscal period validation

- Added tests to validate that mid-month starts are allowed for the first fiscal period.
- Ensured that subsequent periods must start on the 1st of the month and enforced maximum duration constraints.

feat: implement currency rate API endpoint

- Created a new API route to fetch exchange rates for specified currencies, ensuring user authentication.
- Validated currency input and handled errors for invalid requests.

chore: update database constraints for fiscal periods

- Modified database constraints to allow custom start dates for the first fiscal period while enforcing day-1 starts for subsequent periods.

* fix: implement computeFiscalPeriod function for onboarding and refactor JournalEntryForm

* Fixed date issue

* Added migration

lib/core/bookkeeping/period-service.ts

@@ -162,8 +162,8 @@ export async function createNextPeriod(
   const nextStartStr = nextStart.toISOString().split('T')[0]
   const nextEndStr = nextEnd.toISOString().split('T')[0]
 
-  // Validate period duration (max 18 months per BFL 3 kap.)
-  const durationError = validatePeriodDuration(nextStartStr, nextEndStr)
+  // Validate period duration — subsequent periods always start on 1st of month
+  const durationError = validatePeriodDuration(nextStartStr, nextEndStr, { isFirstPeriod: false })
   if (durationError) {
     throw new Error(durationError)
   }