Feature - Add support for a fiscal year period defined in user settings.

* Add "This fiscal year", "Last fiscal year" as date range options in Transaction Details to filter transactions to those periods
* Add fiscal year ranges to Statistics & Trend Analysis
* Add "fiscal year start date" to user profile settings, allowing the user to select any date of the calendar year as the start of the fiscal year
* Add "fiscal year format" to user profile settings, allowing the user to specify how financial year date labels should appear

Implementation notes:
* The default fiscal year start is January 1 and the default fiscal year display format is "FY 2025"
* Fiscal year start date (month number & day number) are stored together in db as a uint16, high byte & low byte respectively
* February 29 is disallowed as a fiscal year start date, since it is never used as a convention in any country
* Jest is added to the project as a dev dependency, for unit tests in frontend

Signed-off-by: Sebastian Reategui <seb.reategui@gmail.com>

pkg/validators/fiscal_year_start_date.go

@@ -0,0 +1,26 @@
+package validators
+
+import (
+	"github.com/gin-gonic/gin/binding"
+	"github.com/go-playground/validator/v10"
+	"github.com/mayswind/ezbookkeeping/pkg/core"
+)
+
+// ValidateFiscalYearStart validates if a fiscal year start date is valid
+func ValidateFiscalYearStart(fl validator.FieldLevel) bool {
+	date, ok := fl.Field().Interface().(core.FiscalYearStart)
+	if !ok {
+		return false
+	}
+
+	// Use the core functionality to validate
+	_, _, err := date.GetMonthDay()
+	return err == nil
+}
+
+// RegisterFiscalYearStartValidator registers the fiscal year start date validator
+func RegisterFiscalYearStartValidator() {
+	if v, ok := binding.Validator.Engine().(*validator.Validate); ok {
+		v.RegisterValidation("validFiscalYearStart", ValidateFiscalYearStart)
+	}
+}

pkg/validators/fiscal_year_start_date_test.go

@@ -0,0 +1,68 @@
+package validators
+
+import (
+	"testing"
+
+	"github.com/go-playground/validator/v10"
+	"github.com/mayswind/ezbookkeeping/pkg/core"
+	"github.com/stretchr/testify/assert"
+)
+
+type fiscalYearStartContainer struct {
+	FiscalYearStart core.FiscalYearStart `validate:"validFiscalYearStart"`
+}
+
+func TestValidateFiscalYearStart_ValidValues(t *testing.T) {
+	validate := validator.New()
+	validate.RegisterValidation("validFiscalYearStart", ValidateFiscalYearStart)
+
+	testCases := []struct {
+		name  string
+		value core.FiscalYearStart
+	}{
+		{"January 1", 0x0101},   // January 1
+		{"December 31", 0x0C1F}, // December 31
+		{"July 1", 0x0701},      // July 1
+		{"April 15", 0x040F},    // April 15
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			container := fiscalYearStartContainer{FiscalYearStart: tc.value}
+			err := validate.Struct(container)
+			assert.Nil(t, err)
+		})
+	}
+}
+
+func TestValidateFiscalYearStart_InvalidValues(t *testing.T) {
+	validate := validator.New()
+	validate.RegisterValidation("validFiscalYearStart", ValidateFiscalYearStart)
+
+	testCases := []struct {
+		name  string
+		value core.FiscalYearStart
+	}{
+		{"Zero value", 0},             // Zero value
+		{"Month 0", 0x0001},           // Month 0 (invalid)
+		{"Month 13", 0x0D01},          // Month 13 (invalid)
+		{"Day 0", 0x0100},             // Day 0 (invalid)
+		{"January 32", 0x0120},        // January 32 (invalid)
+		{"February 29", 0x021D},       // February 29 (not permitted)
+		{"February 30", 0x021E},       // February 30 (invalid)
+		{"April 31", 0x041F},          // April 31 (invalid)
+		{"June 31", 0x061F},           // June 31 (invalid)
+		{"September 31", 0x091F},      // September 31 (invalid)
+		{"November 32", 0x0B20},       // November 32 (invalid)
+		{"Invalid month 255", 0xFF01}, // Invalid month
+		{"Invalid day 255", 0x01FF},   // Invalid day
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			container := fiscalYearStartContainer{FiscalYearStart: tc.value}
+			err := validate.Struct(container)
+			assert.NotNil(t, err)
+		})
+	}
+}