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/bookkeeping/__tests__/validate-period-duration.test.ts

@@ -48,12 +48,42 @@ describe('validatePeriodDuration', () => {
     )
   })
 
-  it('returns error when start is not 1st of month', () => {
+  it('returns error when start is not 1st of month (default)', () => {
     expect(validatePeriodDuration('2025-01-15', '2025-12-31')).toBe(
       'Period start must be the 1st of a month'
     )
   })
 
+  it('returns error when start is not 1st of month (isFirstPeriod: false)', () => {
+    expect(validatePeriodDuration('2025-03-25', '2025-12-31', { isFirstPeriod: false })).toBe(
+      'Period start must be the 1st of a month'
+    )
+  })
+
+  it('allows mid-month start for first fiscal period', () => {
+    expect(validatePeriodDuration('2025-03-25', '2025-12-31', { isFirstPeriod: true })).toBeNull()
+  })
+
+  it('allows mid-month start for first period (October)', () => {
+    expect(validatePeriodDuration('2025-10-15', '2025-12-31', { isFirstPeriod: true })).toBeNull()
+  })
+
+  it('still allows day-1 start for first period', () => {
+    expect(validatePeriodDuration('2025-10-01', '2025-12-31', { isFirstPeriod: true })).toBeNull()
+  })
+
+  it('enforces end-of-month even for first period', () => {
+    expect(validatePeriodDuration('2025-03-25', '2025-12-15', { isFirstPeriod: true })).toBe(
+      'Period end must be the last day of a month'
+    )
+  })
+
+  it('enforces 18-month max for first period with mid-month start', () => {
+    const result = validatePeriodDuration('2025-01-15', '2026-12-31', { isFirstPeriod: true })
+    expect(result).toContain('months')
+    expect(result).toContain('18 months')
+  })
+
   it('returns error when end is not last day of month', () => {
     expect(validatePeriodDuration('2025-01-01', '2025-12-15')).toBe(
       'Period end must be the last day of a month'

lib/bookkeeping/validate-period-duration.ts

@@ -17,7 +17,8 @@ export function parseDateParts(dateStr: string): { year: number; month: number;
 
 /**
  * Calculate the number of months between two dates (inclusive of partial months).
- * Assumes start is 1st of month and end is last of month.
+ * Uses year/month arithmetic only — a mid-month start counts the start month fully,
+ * which is conservative for the 18-month cap check.
  */
 export function monthsBetween(start: string, end: string): number {
   const s = parseDateParts(start)
@@ -25,11 +26,16 @@ export function monthsBetween(start: string, end: string): number {
   return (e.year - s.year) * 12 + (e.month - s.month) + 1
 }
 
+export interface ValidatePeriodOptions {
+  /** Allow any start day (not just 1st of month) for the first fiscal period per BFL 3 kap. */
+  isFirstPeriod?: boolean
+}
+
 /**
  * Validate a fiscal period's duration and date constraints.
  * Returns null if valid, or an error message string if invalid.
  */
-export function validatePeriodDuration(start: string, end: string): string | null {
+export function validatePeriodDuration(start: string, end: string, options?: ValidatePeriodOptions): string | null {
   const startParts = parseDateParts(start)
   const endParts = parseDateParts(end)
 
@@ -38,8 +44,8 @@ export function validatePeriodDuration(start: string, end: string): string | nul
     return 'Period end must be after period start'
   }
 
-  // start must be 1st of month
-  if (startParts.day !== 1) {
+  // start must be 1st of month — unless this is the first fiscal period (BFL 3 kap.)
+  if (startParts.day !== 1 && !options?.isFirstPeriod) {
     return 'Period start must be the 1st of a month'
   }