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/core/fiscalyear_test.go

@@ -0,0 +1,127 @@
+package core
+
+import (
+	"testing"
+
+	"github.com/mayswind/ezbookkeeping/pkg/errs"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestNewFiscalYearStart_ValidMonthDay(t *testing.T) {
+	testCases := []struct {
+		month    uint8
+		day      uint8
+		expected FiscalYearStart
+	}{
+		{1, 1, 0x0101},   // January 1
+		{4, 15, 0x040F},  // April 15
+		{7, 1, 0x0701},   // July 1
+		{12, 31, 0x0C1F}, // December 31
+	}
+
+	for _, tc := range testCases {
+		fiscal, err := NewFiscalYearStart(tc.month, tc.day)
+		assert.Nil(t, err)
+		assert.Equal(t, tc.expected, fiscal)
+	}
+}
+
+func TestNewFiscalYearStart_InvalidMonthDay(t *testing.T) {
+	testCases := []struct {
+		month uint8
+		day   uint8
+	}{
+		{0, 1},    // Month 0 (invalid)
+		{13, 1},   // Month 13 (invalid)
+		{1, 0},    // Day 0 (invalid)
+		{1, 32},   // Day 32 (invalid for January)
+		{2, 30},   // Day 30 (invalid for February)
+		{2, 29},   // Day 29 (leap day not permitted)
+		{4, 31},   // Day 31 (invalid for April)
+		{6, 31},   // Day 31 (invalid for June)
+		{9, 31},   // Day 31 (invalid for September)
+		{11, 32},  // Day 32 (invalid for November)
+		{255, 15}, // Invalid month
+		{5, 255},  // Invalid day
+	}
+
+	for _, tc := range testCases {
+		fiscal, err := NewFiscalYearStart(tc.month, tc.day)
+		assert.Equal(t, FiscalYearStart(0), fiscal)
+		assert.Equal(t, errs.ErrFormatInvalid, err)
+	}
+}
+
+func TestGetMonthDay_ValidFiscalYearStart(t *testing.T) {
+	testCases := []struct {
+		fiscalYear FiscalYearStart
+		month      uint8
+		day        uint8
+	}{
+		{0x0101, 1, 1},   // January 1st
+		{0x0C1F, 12, 31}, // December 31st
+		{0x0701, 7, 1},   // July 1st
+		{0x040F, 4, 15},  // April 15th
+	}
+
+	for _, tc := range testCases {
+		month, day, err := tc.fiscalYear.GetMonthDay()
+		assert.Nil(t, err)
+		assert.Equal(t, tc.month, month)
+		assert.Equal(t, tc.day, day)
+	}
+}
+
+func TestGetMonthDay_InvalidFiscalYearStart(t *testing.T) {
+	testCases := []struct {
+		fiscalYear FiscalYearStart
+	}{
+		{0x0000}, // 0/0 (invalid)
+		{0x0D01}, // Month 13 (invalid)
+		{0x0100}, // Day 0 (invalid)
+		{0x0120}, // January 32 (invalid)
+		{0x021D}, // February 29 (not permitted)
+		{0x021E}, // February 30 (invalid)
+		{0x041F}, // April 31 (invalid)
+		{0x061F}, // June 31 (invalid)
+		{0x091F}, // September 31 (invalid)
+		{0x0B20}, // November 32 (invalid)
+		{0xFF01}, // Invalid month
+		{0x01FF}, // Invalid day
+		{0},      // Zero value
+	}
+
+	for _, tc := range testCases {
+		month, day, err := tc.fiscalYear.GetMonthDay()
+		assert.Equal(t, uint8(0), month)
+		assert.Equal(t, uint8(0), day)
+		assert.Equal(t, errs.ErrFormatInvalid, err)
+	}
+}
+
+func TestFiscalYearStart_String(t *testing.T) {
+	testCases := []struct {
+		fiscalYear FiscalYearStart
+		expected   string
+	}{
+		{0x0101, "01-01"},   // January 1st
+		{0x0C1F, "12-31"},   // December 31st
+		{0x0701, "07-01"},   // July 1st
+		{0x040F, "04-15"},   // April 15th
+		{0x021D, "Invalid"}, // February 29th (leap day not permitted)
+		{0x0000, "Invalid"}, // Invalid date
+		{0x0D01, "Invalid"}, // Invalid month
+		{0x0120, "Invalid"}, // Invalid day
+	}
+
+	for _, tc := range testCases {
+		assert.Equal(t, tc.expected, tc.fiscalYear.String())
+	}
+}
+
+func TestFiscalYearStartConstants(t *testing.T) {
+	assert.Equal(t, FiscalYearStart(0xFFFF), FISCAL_YEAR_START_INVALID)
+	assert.Equal(t, FiscalYearStart(0x0101), FISCAL_YEAR_START_DEFAULT)
+	assert.Equal(t, FiscalYearStart(0x0101), FISCAL_YEAR_START_MIN)
+	assert.Equal(t, FiscalYearStart(0x0C1F), FISCAL_YEAR_START_MAX)
+}