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/handler.go

@@ -24,6 +24,7 @@ const (
 	PendingRate       PendingKind = "rate"
 	PendingCurrency   PendingKind = "currency"
 	PendingTimezone   PendingKind = "timezone"
+	PendingCalendar   PendingKind = "calendar"
 	PendingBreak      PendingKind = "break_threshold"
 	PendingUsername   PendingKind = "username"
 	PendingReportTime PendingKind = "report_time"
@@ -176,6 +177,8 @@ func (h *Handler) routeCommand(ctx context.Context, msg *models.Message, user *d
 		h.handleSetAccent(ctx, msg, user)
 	case "/settimezone":
 		h.handleSetTimezone(ctx, msg, user)
+	case "/setcalendar":
+		h.handleSetCalendar(ctx, msg, user)
 	case "/note":
 		h.handleNote(ctx, msg, user)
 	case "/summary":
@@ -358,6 +361,8 @@ func (h *Handler) processPendingInput(ctx context.Context, msg *models.Message,
 		h.processCurrencyInput(ctx, msg, user, pi, text)
 	case PendingTimezone:
 		h.processTimezoneInput(ctx, msg, user, pi, text)
+	case PendingCalendar:
+		h.processCalendarInput(ctx, msg, user, pi, text)
 	case PendingBreak:
 		h.processBreakInput(ctx, msg, user, pi, text)
 	case PendingUsername: