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/query_agent_last_thread_logic.go

@@ -0,0 +1,40 @@
+// 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 QueryAgentLastThreadLogic struct {
+	logx.Logger
+	ctx    context.Context
+	svcCtx *svc.ServiceContext
+}
+
+func NewQueryAgentLastThreadLogic(ctx context.Context, svcCtx *svc.ServiceContext) *QueryAgentLastThreadLogic {
+	return &QueryAgentLastThreadLogic{
+		Logger: logx.WithContext(ctx),
+		ctx:    ctx,
+		svcCtx: svcCtx,
+	}
+}
+
+func (l *QueryAgentLastThreadLogic) QueryAgentLastThread(req *types.QueryAgentLastThreadRequest) (resp *types.QueryAgentLastThreadResponse, err error) {
+	project, err := l.svcCtx.DesignService.GetProject(l.ctx, req.ProjectId)
+	if err != nil {
+		return nil, err
+	}
+
+	return &types.QueryAgentLastThreadResponse{
+		Code: 0,
+		Msg:  nil,
+		Data: latestLovartThreadID(project),
+	}, nil
+}