feat: stats — per-band mode split (CW / phone / data) stacked bar chart

Adds Stats.ByBandCategory (per band: CW/phone/data counts, band-plan order) via a
modeCategory() bucketing in stats.go, and a StackedBandBars chart in the Stats
panel showing the split per band with a legend.

internal/qso/stats.go

@@ -24,6 +24,30 @@ type Bucket struct {
 	Count int    `json:"count"`
 }
 
+// BandCategory is one band's QSO count split by mode category (CW / phone /
+// digital), for the per-band mode-split chart.
+type BandCategory struct {
+	Band  string `json:"band"`
+	CW    int    `json:"cw"`
+	Phone int    `json:"phone"`
+	Data  int    `json:"data"`
+	Total int    `json:"total"`
+}
+
+// modeCategory buckets a mode into "cw", "phone" or "data" (digital). Voice modes
+// (SSB and the digital-voice family) count as phone; CW is CW; everything else is
+// data. Mirrors the frontend's mode colouring.
+func modeCategory(mode string) string {
+	switch strings.ToUpper(strings.TrimSpace(mode)) {
+	case "CW":
+		return "cw"
+	case "SSB", "USB", "LSB", "AM", "FM", "DV", "DIGITALVOICE", "FREEDV", "C4FM", "DSTAR", "FUSION":
+		return "phone"
+	default:
+		return "data"
+	}
+}
+
 // Gap is a stretch with no QSO at all — the off-air periods. In a contest these
 // are the expensive minutes: they are where the score went.
 type Gap struct {
@@ -129,8 +153,9 @@ type Stats struct {
 	ConfirmedAny  int `json:"confirmed_any"`
 
 	// Breakdowns, each sorted most → least (bands keep frequency order).
-	ByMode      []Bucket `json:"by_mode"`
-	ByBand      []Bucket `json:"by_band"`
+	ByMode         []Bucket       `json:"by_mode"`
+	ByBand         []Bucket       `json:"by_band"`
+	ByBandCategory []BandCategory `json:"by_band_category"` // per band: CW / phone / data split
 	ByOperator  []Bucket `json:"by_operator"`
 	ByStation   []Bucket `json:"by_station"` // station_callsign (the call put on the air)
 	ByContinent []Bucket `json:"by_continent"`
@@ -268,6 +293,7 @@ func (r *Repo) Stats(ctx context.Context, from, to time.Time, contestID string,
 		entities   = map[int]struct{}{}
 		modeC      = map[string]int{}
 		bandC      = map[string]int{}
+		bandCat    = map[string]*BandCategory{} // per band: cw/phone/data
 		opC        = map[string]int{}
 		stationC   = map[string]int{}
 		contC      = map[string]int{}
@@ -338,6 +364,20 @@ func (r *Repo) Stats(ctx context.Context, from, to time.Time, contestID string,
 		}
 		if b := strings.ToLower(strings.TrimSpace(band.String)); b != "" {
 			bandC[b]++
+			bc := bandCat[b]
+			if bc == nil {
+				bc = &BandCategory{Band: b}
+				bandCat[b] = bc
+			}
+			switch modeCategory(mode.String) {
+			case "cw":
+				bc.CW++
+			case "phone":
+				bc.Phone++
+			default:
+				bc.Data++
+			}
+			bc.Total++
 		}
 		// op was resolved above (with the operator filter applied).
 		opC[op]++
@@ -414,6 +454,13 @@ func (r *Repo) Stats(ctx context.Context, from, to time.Time, contestID string,
 		}
 		return a < b
 	})
+	// Per-band CW/phone/data split, in the SAME band-plan order as ByBand.
+	s.ByBandCategory = []BandCategory{}
+	for _, b := range s.ByBand {
+		if bc := bandCat[b.Key]; bc != nil {
+			s.ByBandCategory = append(s.ByBandCategory, *bc)
+		}
+	}
 	// The time axis must be CONTINUOUS. Emitting only the months that have QSOs
 	// would place, say, 2012-08 next to 2022-01 as if they were consecutive — the
 	// chart would invent activity that never happened. A gap in the log is real