fix(decodes): a decode's mode is a marker, not a mode name

Every station on an already-worked band was flagged NEW MODE.

A WSJT-X Decode does not carry the mode's name. It carries the
one-character marker from the decode line - "~" for FT8, "+" for FT4 - and
that character was passed straight through as though it were a mode. The
status resolver then compared "~" against the modes worked for the entity,
matched nothing, and concluded the mode had never been worked. Same cause
put "~ -07" in the comment of every decode spot pushed to the FlexRadio
panadapter, which nobody had traced back.

Resolved through a marker table, with the mode from the sender's last
Status as the fallback - Status is the message that carries the real name.
So an unlisted or future marker degrades to correct rather than to
nonsense, and a sender that puts the name in the field directly is believed
as-is. With neither available the mode is left empty, which makes the
resolver answer "worked": the safe side, since a wrong mode invents a
new-mode flag exactly as the marker did.

internal/integrations/udp/wsjt.go

@@ -453,3 +453,50 @@ func readQString(r *bytes.Reader) (string, error) {
 	}
 	return string(buf), nil
 }
+
+// decodeModeChar maps the single character WSJT-X puts in a Decode's mode field
+// to the mode it stands for.
+//
+// A Decode does NOT carry the mode's name. It carries the one-character marker
+// that appears in the decode line and in ALL.TXT — "~" for FT8, "+" for FT4 —
+// and that character was being passed straight through as if it were a mode.
+// Everything downstream then compared "~" against the modes in the log, matched
+// nothing, and called every station on an already-worked band a new MODE.
+//
+// The table covers what is common; anything missing falls back to the mode from
+// the sender's last Status, which carries the real name — so an unlisted or
+// future marker degrades to correct rather than to nonsense.
+var decodeModeChar = map[string]string{
+	"~": "FT8",
+	"+": "FT4",
+	"#": "JT65",
+	"@": "JT9",
+	"&": "MSK144",
+	":": "Q65",
+	"`": "FST4",
+}
+
+// DecodeModeName resolves a Decode's mode field to a real mode name. statusMode
+// is the mode from the same program's last Status, used when the field is a
+// marker we do not know, or empty.
+func DecodeModeName(raw, statusMode string) string {
+	raw = strings.TrimSpace(raw)
+	if m, ok := decodeModeChar[raw]; ok {
+		return m
+	}
+	// A mode name is at least two alphanumeric characters ("FT8", "JS8", "Q65").
+	// Anything shorter, or carrying punctuation, is a marker rather than a name.
+	if len(raw) >= 2 {
+		named := true
+		for _, r := range raw {
+			if !(r >= 'A' && r <= 'Z') && !(r >= 'a' && r <= 'z') && !(r >= '0' && r <= '9') {
+				named = false
+				break
+			}
+		}
+		if named {
+			return strings.ToUpper(raw)
+		}
+	}
+	return strings.ToUpper(strings.TrimSpace(statusMode))
+}