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/lovart_agent_mapper.go

@@ -0,0 +1,78 @@
+package logic
+
+import (
+	"strings"
+	"time"
+
+	"img_infinite_canvas/internal/domain/design"
+	"img_infinite_canvas/internal/types"
+)
+
+func latestLovartThreadID(project design.Project) string {
+	if threadID := strings.TrimSpace(project.LastThreadID); threadID != "" {
+		return threadID
+	}
+	for index := len(project.Messages) - 1; index >= 0; index-- {
+		message := project.Messages[index]
+		if isHiddenAgentMessage(message) {
+			continue
+		}
+		if threadID := strings.TrimSpace(message.ThreadID); threadID != "" {
+			return threadID
+		}
+	}
+	return ""
+}
+
+func toCandaChatHistoryItem(message design.Message, projectID string, threadID string) types.CandaChatHistoryItem {
+	messageType := strings.TrimSpace(message.Type)
+	if messageType == "" {
+		messageType = strings.TrimSpace(message.Role)
+	}
+	if messageType == "tool" {
+		messageType = "tool_use"
+	}
+	item := types.CandaChatHistoryItem{
+		Id:         message.ID,
+		Timestamp:  formatCandaTimestamp(message.CreatedAt),
+		Type:       messageType,
+		ActionId:   nonEmpty(message.ActionID, threadID),
+		StepId:     nonEmpty(message.StepID, message.ID),
+		Status:     message.Status,
+		ToolCallId: message.ToolCallID,
+		Name:       message.Name,
+		ThreadMeta: types.CandaThreadMeta{
+			ActionId:  nonEmpty(message.ActionID, threadID),
+			SessionId: threadID + "_main",
+			StepId:    nonEmpty(message.StepID, message.ID),
+			Depth:     0,
+			AgentName: "next_agent",
+			ProjectId: projectID,
+		},
+	}
+	if message.Role == "user" {
+		item.Text = message.Content
+	} else {
+		item.Content = message.Content
+		if item.Name == "" {
+			item.Name = message.Title
+		}
+	}
+	return item
+}
+
+func nonEmpty(values ...string) string {
+	for _, value := range values {
+		if strings.TrimSpace(value) != "" {
+			return value
+		}
+	}
+	return ""
+}
+
+func formatCandaTimestamp(value time.Time) string {
+	if value.IsZero() {
+		value = time.Now()
+	}
+	return value.UTC().Format(time.RFC3339Nano)
+}