fix: currency validation, league mobile layout, settings 2-col, burnout ratio, add setusername

- Require both symbol+code in /setcurrency; add preset picker ($ USD,
  T Toman, IRR Rial, EUR, GBP) in settings inline menu
- League: compact 2-line per entry for mobile; sort buttons in 2x2 grid
- Settings: 2-column layout with grouped buttons (timezone+calendar,
  report+report time, break+accent, rpg+league, currency+rate)
- Burnout: cap break ratio at 100% to prevent nonsense values
- Add /setusername command (uses existing SanitizeDisplayName)
- Update /help with new/changed commands

internal/bot/burnout.go

@@ -258,6 +258,9 @@ func (h *Handler) buildBurnoutText(userID int64, loc *time.Location) string {
 		br := float64(res.BreakSeconds)
 		total := float64(res.WorkSeconds + res.BreakSeconds)
 		ratio := br / total * 100
+		if ratio > 100 {
+			ratio = 100
+		}
 		b.WriteString(fmt.Sprintf("  Break ratio: %.0f%% (%d pts)\n", ratio, res.BreakRatioPts))
 	}
 	if res.WeekendPts > 0 {