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

supabase/migrations/20260409165300_allow_custom_first_fiscal_period_start.sql

@@ -0,0 +1,36 @@
+-- Allow the first fiscal period of a company to start on any day of the month,
+-- per BFL 3 kap. (the first fiscal year starts on the company registration date).
+-- Subsequent periods must still start on the 1st of a month.
+
+-- Drop the unconditional CHECK constraint that enforces day-1 starts
+ALTER TABLE public.fiscal_periods
+  DROP CONSTRAINT IF EXISTS fiscal_period_start_first_of_month;
+
+-- Replace with a trigger that only enforces day-1 for non-first periods
+CREATE OR REPLACE FUNCTION enforce_first_of_month_for_subsequent_periods()
+RETURNS trigger AS $$
+BEGIN
+  -- If this period starts on the 1st, no check needed
+  IF EXTRACT(DAY FROM NEW.period_start) = 1 THEN
+    RETURN NEW;
+  END IF;
+
+  -- Allow any start day only if this is the first fiscal period for the company
+  IF EXISTS (
+    SELECT 1 FROM public.fiscal_periods
+    WHERE company_id = NEW.company_id
+      AND id IS DISTINCT FROM NEW.id
+  ) THEN
+    RAISE EXCEPTION 'Non-first fiscal period must start on the 1st of a month';
+  END IF;
+
+  RETURN NEW;
+END;
+$$ LANGUAGE plpgsql;
+
+DROP TRIGGER IF EXISTS enforce_period_start_day ON public.fiscal_periods;
+
+CREATE TRIGGER enforce_period_start_day
+  BEFORE INSERT OR UPDATE ON public.fiscal_periods
+  FOR EACH ROW
+  EXECUTE FUNCTION enforce_first_of_month_for_subsequent_periods();