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/constants.go

@@ -0,0 +1,19 @@
+package domain
+
+import "time"
+
+const (
+	DateLayout            = "2006-01-02"
+	TimeLayout            = "15:04"
+	SecondsPerHour        = 3600
+	SecondsPerDay         = 86400
+	DefaultBreakThreshold = 300
+	MaxBreakThresholdMins = 480
+	StreakBonusCap        = 500
+	XPCurveMultiplier     = 50
+	SalaryRoundFactor     = 100
+	MaxHourlyRate         = 999999
+	RateLimitInterval     = 1 * time.Second
+	MaxNoteLength         = 500
+	MaxUsernameLength     = 64
+)