feat(api): add Lovart-style project and agent thread endpoints

Add canva/canda-compatible routes for lightweight project open/save and
agent thread history:
- POST /canva/project/queryProject, saveProject
- POST /canva/agent/queryAgentLastThread, agentThreadList
- GET /canda/chat-history, /canda/thread/status

Canvas save now accepts a compressed SHAKKERDATA:// snapshot payload,
preserves connections, and returns a SaveProject response; blank
successful image nodes are backfilled from generated artifacts. Frontend
designGateway adopts the new save/query flow.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/internal/logic/canda_thread_status_logic.go

@@ -0,0 +1,44 @@
+// Code scaffolded by goctl. Safe to edit.
+// goctl 1.10.1
+
+package logic
+
+import (
+	"context"
+
+	"img_infinite_canvas/internal/svc"
+	"img_infinite_canvas/internal/types"
+
+	"github.com/zeromicro/go-zero/core/logx"
+)
+
+type CandaThreadStatusLogic struct {
+	logx.Logger
+	ctx    context.Context
+	svcCtx *svc.ServiceContext
+}
+
+func NewCandaThreadStatusLogic(ctx context.Context, svcCtx *svc.ServiceContext) *CandaThreadStatusLogic {
+	return &CandaThreadStatusLogic{
+		Logger: logx.WithContext(ctx),
+		ctx:    ctx,
+		svcCtx: svcCtx,
+	}
+}
+
+func (l *CandaThreadStatusLogic) CandaThreadStatus(req *types.CandaThreadStatusRequest) (resp *types.CandaThreadStatusResponse, err error) {
+	status := "done"
+	if req.ProjectId != "" {
+		thread, err := l.svcCtx.DesignService.AgentThreadStatus(l.ctx, req.ProjectId)
+		if err != nil {
+			return nil, err
+		}
+		status = thread.Status
+	}
+
+	return &types.CandaThreadStatusResponse{
+		Code: 0,
+		Msg:  "success",
+		Data: types.CandaThreadStatusData{Status: status},
+	}, nil
+}