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/application/image_generator_target_test.go

@@ -119,6 +119,48 @@ func TestRestoreFinishedGeneratedImageNodesRepairsStaleCanvasSnapshot(t *testing
 	}
 }
 
+func TestRestoreFinishedGeneratedImageNodesRepairsBlankSuccessfulImageNode(t *testing.T) {
+	now := time.Now()
+	project := design.Project{
+		ID:     "project-blank-generated-node",
+		Status: design.StatusReady,
+		Nodes: []design.Node{
+			{
+				ID:     "blank-node",
+				Type:   design.NodeTypeImage,
+				Status: "success",
+				Width:  320,
+				Height: 240,
+			},
+		},
+		Messages: []design.Message{
+			toolArtifactsMessage("thread-image", "pants", []design.Node{
+				{
+					ID:      "artifact-node",
+					Type:    design.NodeTypeImage,
+					Title:   "浅麻灰长裤抠图",
+					Content: "http://localhost:19000/canvas-assets/pants.png",
+					Status:  "success",
+					Width:   1024,
+					Height:  1024,
+				},
+			}, now),
+		},
+	}
+
+	restored, changed := (*DesignService)(nil).restoreFinishedGeneratedImageNodes(project, now.Add(time.Second))
+	if !changed {
+		t.Fatal("expected blank generated node to be repaired")
+	}
+	if len(restored.Nodes) != 1 {
+		t.Fatalf("expected one repaired node, got %#v", restored.Nodes)
+	}
+	node := restored.Nodes[0]
+	if node.ID != "blank-node" || node.Content != "http://localhost:19000/canvas-assets/pants.png" || node.Title != "浅麻灰长裤抠图" || node.Width != 1024 || node.Height != 1024 {
+		t.Fatalf("expected blank node repaired from artifact, got %#v", node)
+	}
+}
+
 func TestMergeSavedCanvasWorkStateDoesNotReapplyStaleGeneratingNodes(t *testing.T) {
 	existing := []design.Node{
 		{ID: "generated", Type: design.NodeTypeImage, Title: "Generated", Content: "http://localhost:19000/canvas-assets/generated.png", Status: "success"},