feat(dataexchange): add data_b64 field to inbox messages

Store the raw payload as standard base64 alongside the existing
`data` string field. The string representation mangles bytes > 0x7F
(Go's json.Marshal replaces invalid UTF-8 with U+FFFD), making
binary payloads (zlib-compressed HealthKit envelopes, etc.) impossible
to recover. `data_b64` is the lossless counterpart.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: teovl

plugins/dataexchange/service.go

@@ -7,6 +7,7 @@ package dataexchange
 
 import (
 	"context"
+	"encoding/base64"
 	"encoding/json"
 	"fmt"
 	"log/slog"
@@ -239,6 +240,7 @@ func (s *Service) saveInboxMessage(frame *internaldx.Frame, from protocol.Addr)
 		"type":        internaldx.TypeName(frame.Type),
 		"from":        from.String(),
 		"data":        string(frame.Payload),
+		"data_b64":    base64.StdEncoding.EncodeToString(frame.Payload),
 		"bytes":       len(frame.Payload),
 		"received_at": ts.Format(time.RFC3339Nano),
 	}