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/handler/routes.go

@@ -94,6 +94,36 @@ func RegisterHandlers(server *rest.Server, serverCtx *svc.ServiceContext) {
 				Path:    "/auth/options",
 				Handler: getAuthOptionsHandler(serverCtx),
 			},
+			{
+				Method:  http.MethodGet,
+				Path:    "/canda/chat-history",
+				Handler: candaChatHistoryHandler(serverCtx),
+			},
+			{
+				Method:  http.MethodGet,
+				Path:    "/canda/thread/status",
+				Handler: candaThreadStatusHandler(serverCtx),
+			},
+			{
+				Method:  http.MethodPost,
+				Path:    "/canva/agent/agentThreadList",
+				Handler: agentThreadListHandler(serverCtx),
+			},
+			{
+				Method:  http.MethodPost,
+				Path:    "/canva/agent/queryAgentLastThread",
+				Handler: queryAgentLastThreadHandler(serverCtx),
+			},
+			{
+				Method:  http.MethodPost,
+				Path:    "/canva/project/queryProject",
+				Handler: queryProjectHandler(serverCtx),
+			},
+			{
+				Method:  http.MethodPost,
+				Path:    "/canva/project/saveProject",
+				Handler: saveProjectHandler(serverCtx),
+			},
 			{
 				Method:  http.MethodPost,
 				Path:    "/context/vision/recognize_object",