feat(group): add SetNotFoundHandle support for router groups

[devfeel]

Summary

• Add SetNotFoundHandle method to Group interface
• Add notFoundHandler field to xGroup struct
• Group-level NotFoundHandler takes priority over app-level handler

Usage Example

func main() {
    dotapp := dotweb.New()
    
    // Create API group
    apiGroup := dotapp.HttpServer.Group("/api")
    
    // Set group-specific 404 handler
    apiGroup.SetNotFoundHandle(func(ctx dotweb.Context) error {
        ctx.WriteString("API 404 - Resource not found")
        return nil
    })
    
    // Register routes
    apiGroup.GET("/users", func(ctx dotweb.Context) error {
        ctx.WriteString("Users list")
        return nil
    })
    
    dotapp.StartServer(8080)
}

Behavior

• Request to /api/users → Returns "Users list"
• Request to /api/unknown → Returns "API 404 - Resource not found" (group handler)
• Request to /unknown → Returns global 404 (app handler)

Test Plan

• All existing tests pass

• Manual testing with example code

• Verify group 404 handler priority

• Verify fallback to app-level handler

Resolves: #229