fix: enforce Rule 1 JSON wrappers across 80+ files, fix 6 bugs

- Replace all encoding/json direct calls with common.Marshal/Unmarshal/DecodeJson per Rule 1
- Fix Dify nil pointer dereference on remote image upload (relay-dify.go)
- Fix Claude relay file content type detection for text/* and PDF (relay-claude.go)
- Fix unsafe type assertions in Claude relay and Vertex GetModelRegion
- Fix StreamScanner unconditionally resetting pre-existing StreamStatus
- Add inferMimeTypeFromFilename() for proper MIME type handling in DTO
- Fix Mac build script hardcoded DMG version (now reads from tauri.conf.json)

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

heicode/service/webhook.go

@@ -5,7 +5,6 @@ import (
 	"crypto/hmac"
 	"crypto/sha256"
 	"encoding/hex"
-	"encoding/json"
 	"fmt"
 	"net/http"
 	"time"
@@ -49,7 +48,7 @@ func SendWebhookNotify(webhookURL string, secret string, data dto.Notify) error
 	}
 
 	// 序列化负载
-	payloadBytes, err := json.Marshal(payload)
+	payloadBytes, err := common.Marshal(payload)
 	if err != nil {
 		return fmt.Errorf("failed to marshal webhook payload: %v", err)
 	}