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.go

@@ -0,0 +1,109 @@
+package core
+
+import (
+	"fmt"
+
+	"github.com/mayswind/ezbookkeeping/pkg/errs"
+)
+
+// FiscalYearStart represents the fiscal year start date as a uint16 (month: high byte, day: low byte)
+type FiscalYearStart uint16
+
+// Fiscal Year Start Date Type
+const (
+	FISCAL_YEAR_START_DEFAULT FiscalYearStart = 0x0101 // January 1
+	FISCAL_YEAR_START_MIN     FiscalYearStart = 0x0101 // January 1
+	FISCAL_YEAR_START_MAX     FiscalYearStart = 0x0C1F // December 31
+	FISCAL_YEAR_START_INVALID FiscalYearStart = 0xFFFF // Invalid
+)
+
+// NewFiscalYearStart creates a new FiscalYearStart from month and day values
+func NewFiscalYearStart(month uint8, day uint8) (FiscalYearStart, error) {
+	month, day, err := validateMonthDay(month, day)
+	if err != nil {
+		return 0, err
+	}
+
+	return FiscalYearStart(uint16(month)<<8 | uint16(day)), nil
+}
+
+// GetMonthDay extracts the month and day from FiscalYearType
+func (f FiscalYearStart) GetMonthDay() (uint8, uint8, error) {
+	if f < FISCAL_YEAR_START_MIN || f > FISCAL_YEAR_START_MAX {
+		return 0, 0, errs.ErrFormatInvalid
+	}
+
+	// Extract month and day (month in high byte, day in low byte)
+	month := uint8(f >> 8)
+	day := uint8(f & 0xFF)
+
+	return validateMonthDay(month, day)
+}
+
+// String returns a string representation of FiscalYearStart in MM/DD format
+func (f FiscalYearStart) String() string {
+	month, day, err := f.GetMonthDay()
+	if err != nil {
+		return "Invalid"
+	}
+	return fmt.Sprintf("%02d-%02d", month, day)
+}
+
+// validateMonthDay validates a month and day and returns them if valid
+func validateMonthDay(month uint8, day uint8) (uint8, uint8, error) {
+	if month < 1 || month > 12 || day < 1 {
+		return 0, 0, errs.ErrFormatInvalid
+	}
+
+	maxDays := uint8(31)
+	switch month {
+	case 1, 3, 5, 7, 8, 10, 12: // January, March, May, July, August, October, December
+		maxDays = 31
+	case 4, 6, 9, 11: // April, June, September, November
+		maxDays = 30
+	case 2: // February
+		maxDays = 28 // Disallow fiscal year start on leap day
+	}
+
+	if day > maxDays {
+		return 0, 0, errs.ErrFormatInvalid
+	}
+
+	return month, day, nil
+}
+
+// FiscalYearFormat represents the fiscal year format as a uint8
+type FiscalYearFormat uint8
+
+// Fiscal Year Format Type Name
+const (
+	FISCAL_YEAR_FORMAT_DEFAULT           FiscalYearFormat = 0
+	FISCAL_YEAR_FORMAT_STARTYYYY_ENDYYYY FiscalYearFormat = 1
+	FISCAL_YEAR_FORMAT_STARTYYYY_ENDYY   FiscalYearFormat = 2
+	FISCAL_YEAR_FORMAT_STARTYY_ENDYY     FiscalYearFormat = 3
+	FISCAL_YEAR_FORMAT_ENDYYYY           FiscalYearFormat = 4
+	FISCAL_YEAR_FORMAT_ENDYY             FiscalYearFormat = 5
+	FISCAL_YEAR_FORMAT_INVALID           FiscalYearFormat = 255 // Invalid
+)
+
+// String returns a textual representation of the long date format enum
+func (f FiscalYearFormat) String() string {
+	switch f {
+	case FISCAL_YEAR_FORMAT_DEFAULT:
+		return "Default"
+	case FISCAL_YEAR_FORMAT_STARTYYYY_ENDYYYY:
+		return "StartYYYY-EndYYYY"
+	case FISCAL_YEAR_FORMAT_STARTYYYY_ENDYY:
+		return "StartYYYY-EndYY"
+	case FISCAL_YEAR_FORMAT_STARTYY_ENDYY:
+		return "StartYY-EndYY"
+	case FISCAL_YEAR_FORMAT_ENDYYYY:
+		return "EndYYYY"
+	case FISCAL_YEAR_FORMAT_ENDYY:
+		return "EndYY"
+	case FISCAL_YEAR_FORMAT_INVALID:
+		return "Invalid"
+	default:
+		return fmt.Sprintf("Invalid(%d)", int(f))
+	}
+}