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/save_canvas_logic.go

@@ -6,6 +6,7 @@ package logic
 import (
 	"context"
 
+	"img_infinite_canvas/internal/domain/design"
 	"img_infinite_canvas/internal/svc"
 	"img_infinite_canvas/internal/types"
 
@@ -26,7 +27,7 @@ func NewSaveCanvasLogic(ctx context.Context, svcCtx *svc.ServiceContext) *SaveCa
 	}
 }
 
-func (l *SaveCanvasLogic) SaveCanvas(req *types.SaveCanvasRequest) (resp *types.ProjectResponse, err error) {
+func (l *SaveCanvasLogic) SaveCanvas(req *types.SaveCanvasRequest) (resp *types.SaveProjectResponse, err error) {
 	project, err := l.svcCtx.DesignService.SaveCanvas(
 		l.ctx,
 		req.Id,
@@ -41,5 +42,18 @@ func (l *SaveCanvasLogic) SaveCanvas(req *types.SaveCanvasRequest) (resp *types.
 		return nil, err
 	}
 
-	return toProjectResponse(project), nil
+	return saveProjectResponse(project), nil
+}
+
+func saveProjectResponse(project design.Project) *types.SaveProjectResponse {
+	return &types.SaveProjectResponse{
+		Code: 0,
+		Msg:  nil,
+		Data: types.SaveProjectData{
+			ProjectId:      project.ID,
+			NewCreated:     false,
+			ValidProjectId: true,
+			Version:        project.Version,
+		},
+	}
 }