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/rpg.go

@@ -0,0 +1,76 @@
+package domain
+
+import "time"
+
+// XpForLevel returns the total XP required to reach level n.
+func XpForLevel(n int) int64 {
+	return int64(n) * int64(n+1) * XPCurveMultiplier
+}
+
+// LevelForXP returns the level for a given total XP amount.
+func LevelForXP(xp int64) int {
+	l := 1
+	for XpForLevel(l) <= xp {
+		l++
+	}
+	return l - 1
+}
+
+// XpForWorkSeconds calculates XP earned for a given amount of work.
+func XpForWorkSeconds(sec int64) int64 {
+	return sec
+}
+
+// StreakBonus returns bonus XP for maintaining a streak.
+func StreakBonus(streak int) int64 {
+	b := int64(streak) * XPCurveMultiplier
+	if b > StreakBonusCap {
+		b = StreakBonusCap
+	}
+	return b
+}
+
+// ComputeStreakFromDates computes current and longest streak from a sorted list of date strings.
+func ComputeStreakFromDates(dates []string, today string) (current, longest int) {
+	if len(dates) == 0 {
+		return 0, 0
+	}
+
+	dateSet := make(map[string]struct{}, len(dates))
+	for _, d := range dates {
+		dateSet[d] = struct{}{}
+	}
+
+	if _, ok := dateSet[today]; ok {
+		current = 1
+		for i := 1; ; i++ {
+			t, _ := time.Parse(DateLayout, today)
+			prev := t.AddDate(0, 0, -i).Format(DateLayout)
+			if _, ok := dateSet[prev]; ok {
+				current++
+			} else {
+				break
+			}
+		}
+	}
+
+	longest = 0
+	run := 1
+	for i := 1; i < len(dates); i++ {
+		prev, _ := time.Parse(DateLayout, dates[i-1])
+		curr, _ := time.Parse(DateLayout, dates[i])
+		if curr.Sub(prev).Hours() == 24 {
+			run++
+		} else {
+			if run > longest {
+				longest = run
+			}
+			run = 1
+		}
+	}
+	if run > longest {
+		longest = run
+	}
+
+	return current, longest
+}