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/burnout.go

@@ -0,0 +1,24 @@
+package domain
+
+import (
+	"fmt"
+	"math"
+)
+
+// ComputeTrendFromAvgs is a pure version of the trend computation for testing.
+func ComputeTrendFromAvgs(recentAvg, olderAvg float64) (pts int, direction string) {
+	if olderAvg < 1 {
+		if recentAvg > 0 {
+			return 15, "new"
+		}
+		return 5, "stable"
+	}
+	changePct := math.Round((recentAvg - olderAvg) / olderAvg * 100)
+	if recentAvg > olderAvg*1.2 {
+		return 15, fmt.Sprintf("+%.0f%%", changePct)
+	}
+	if olderAvg > recentAvg*1.2 {
+		return 0, fmt.Sprintf("%.0f%%", changePct)
+	}
+	return 5, "stable"
+}