Refactor web UI server setup in main.go and update default host in config

src/webui/README.md

@@ -0,0 +1,108 @@
+# WebUI Module
+
+This module provides a web interface for managing Yggdrasil node through a browser.
+
+## Features
+
+- ✅ HTTP web server with static files
+- ✅ Health check endpoint (`/health`)
+- ✅ Development and production build modes
+- ✅ Automatic binding to Yggdrasil IPv6 address
+- ✅ IPv4 and IPv6 support
+- ✅ Path traversal attack protection
+
+## Configuration
+
+In the Yggdrasil configuration file:
+
+```json
+{
+  "WebUI": {
+    "Enable": true,
+    "Port": 9000,
+    "Host": "",
+    "BindYgg": false
+  }
+}
+```
+
+### Configuration parameters:
+
+- **`Enable`** - enable/disable WebUI
+- **`Port`** - port for web interface (default 9000)
+- **`Host`** - IP address to bind to (empty means all interfaces)
+- **`BindYgg`** - automatically bind to Yggdrasil IPv6 address
+
+## Usage
+
+### Standard mode
+
+```go
+server := webui.Server("127.0.0.1:9000", logger)
+```
+
+### With core access
+
+```go
+server := webui.ServerWithCore("127.0.0.1:9000", logger, coreInstance)
+```
+
+### Automatic Yggdrasil address binding
+
+```go
+server := webui.ServerForYggdrasil(9000, logger, coreInstance)
+// Automatically binds to [yggdrasil_ipv6]:9000
+```
+
+### Starting the server
+
+```go
+go func() {
+    if err := server.Start(); err != nil {
+        logger.Errorf("WebUI server error: %v", err)
+    }
+}()
+
+// To stop
+server.Stop()
+```
+
+## Endpoints
+
+- **`/`** - main page (index.html)
+- **`/health`** - health check (returns "OK")
+- **`/static/*`** - static files (CSS, JS, images)
+
+## Build modes
+
+### Development mode (`-tags debug`)
+- Files loaded from disk from `src/webui/static/`
+- File changes available without rebuild
+
+### Production mode (default)
+- Files embedded in binary
+- Faster loading, smaller deployment size
+
+## Security
+
+- Path traversal attack protection
+- Configured HTTP timeouts
+- Header size limits
+- File MIME type validation
+
+## Testing
+
+The module includes a comprehensive test suite:
+
+```bash
+cd src/webui
+go test -v
+```
+
+Tests cover:
+- Server creation and management
+- HTTP endpoints
+- Static files (dev and prod modes)
+- Error handling
+- Configuration
+- Yggdrasil IPv6 binding