feat(mcp): add diagram version history

- Add history.ts module with circular buffer (max 50 entries)
- Add history UI with floating button and modal
- Add HTTP endpoints: /api/history, /api/restore
- Add MCP tools: list_history, restore_version, get_version
- Save history before and after AI changes
- Track source (ai/human) for each entry

.mcp.json

@@ -0,0 +1,10 @@
+{
+    "mcpServers": {
+        "drawio": {
+            "command": "node",
+            "args": [
+                "/Users/jiangdy/Documents/programming/next-ai-draw-io/packages/mcp-server/dist/index.js"
+            ]
+        }
+    }
+}