feat: add /setcalendar command, store tests, and improve error handling

- Add /setcalendar command for calendar type parity (Settings UI)
- Add 17 store tests covering all repository methods
- Add handler tests for formatDuration, fmtHHMM, fmtDDHHMM
- Add domain tests for BurnoutLevel.String, NavMonth, FormatSalary
- Fix silent error discarding in save* helpers (log write failures)
- Fix computeAndAwardXP error handling instead of discarding
- Wrap critical store.go SQL errors with fmt.Errorf context
- Update README commands table with /setcalendar

internal/handler/rpg.go

@@ -134,12 +134,15 @@ func (h *Handler) buildAchievementsList(user *domain.User) string {
 }
 
 func (h *Handler) awardXPAndCheckAchievements(user *domain.User, stats *domain.RPGStats, sessionWork int64, today string, isWeekend bool) string {
-	text := ""
 	h.updateStreak(stats, user.ID, today)
-	xp, leveledUp, newLevel, _ := h.computeAndAwardXP(stats, user.ID, sessionWork, stats.CurrentStreak, today)
+	xp, leveledUp, newLevel, err := h.computeAndAwardXP(stats, user.ID, sessionWork, stats.CurrentStreak, today)
+	if err != nil {
+		slog.Error("compute and award xp failed", "user_id", user.ID, "error", err)
+		return ""
+	}
 	totalHours := float64(stats.TotalWorkSeconds) / domain.SecondsPerHour
 	unlocked := h.checkAchievements(user.ID, stats, sessionWork, today, isWeekend, totalHours)
-	text += fmt.Sprintf("\n\n+%d XP (Lv.%d)", xp, newLevel)
+	text := fmt.Sprintf("\n\n+%d XP (Lv.%d)", xp, newLevel)
 	if leveledUp {
 		text += "\nLevel up!"
 	}