fix: resolve go vet error and dayStart logic in ComputeDailyTotals

- Fix 'string(d)' int-to-rune conversion in rpg_test.go by using fmt.Sprintf
- Fix ComputeDailyTotals dayStart logic: initial state should be StateWorking
  when dayStart > 0, correctly counting work from dayStart to first out event
- Update README project structure to reflect new domain/handler/repo layout
- Implement missing achievements: rpg_pioneer (on RPG enable) and
  salary_setter (on first rate set)
- Add tryUnlockAchievement helper to Handler

internal/domain/salary.go

@@ -0,0 +1,51 @@
+package domain
+
+import (
+	"fmt"
+	"math"
+	"strings"
+	"unicode"
+)
+
+// ComputeDailySalary calculates salary for a single day's work.
+func ComputeDailySalary(workSeconds int64, hourlyRate float64) float64 {
+	hours := float64(workSeconds) / SecondsPerHour
+	return math.Round(hours*hourlyRate*SalaryRoundFactor) / SalaryRoundFactor
+}
+
+// CurrencySymbol extracts the first token as symbol, defaulting to "$".
+func CurrencySymbol(currency string) string {
+	if parts := strings.Fields(currency); len(parts) >= 1 && parts[0] != "" {
+		return parts[0]
+	}
+	return "$"
+}
+
+// FormatSalary formats a salary estimate into a readable string.
+func (e SalaryEstimate) FormatSalary(currency string) string {
+	sym := CurrencySymbol(currency)
+	code := "USD"
+	if parts := strings.Fields(currency); len(parts) >= 2 {
+		code = parts[1]
+	}
+	return fmt.Sprintf("%s%.2f %s", sym, e.GrossEarning, code)
+}
+
+// FormatSalaryShort returns just the symbol + amount.
+func (e SalaryEstimate) FormatSalaryShort(currency string) string {
+	return fmt.Sprintf("%s%.2f", CurrencySymbol(currency), e.GrossEarning)
+}
+
+// ParseCurrencyInput validates and normalizes currency input (symbol + code).
+func ParseCurrencyInput(s string) (string, string) {
+	parts := strings.Fields(s)
+	if len(parts) < 2 {
+		return "", "Please provide both symbol and code.\nUsage: /setcurrency <symbol> <code>\nExamples:\n/setcurrency $ USD\n/setcurrency T Toman\n/setcurrency IRR Rial"
+	}
+	for _, p := range parts[:2] {
+		if len(p) > 10 || strings.ContainsFunc(p, unicode.IsControl) {
+			return "", "Invalid currency. Use short printable strings like $ USD, T Toman, IRR Rial."
+		}
+	}
+	return parts[0] + " " + parts[1], ""
+}