chore: 初始提交 Fonu 1.0.2

- 公网反代：Nginx 状态卡片（版本 / 运行状态 / 配置校验 / 端口监听）
- 部署设置第一、二梯队（HTTPS 开关、安全头、限流、连接数、仅中国大陆 IP）
- 隧道检测兼容 frps v0.68 / v0.71（V2 与旧版端点）
- 恢复被清空的 internal/frp/store.go、internal/api/ddns_handler.go、web/src/views/DdnsView.vue

internal/api/response.go

@@ -0,0 +1,64 @@
+package api
+
+import (
+	"encoding/json"
+	"log/slog"
+	"net/http"
+	"reflect"
+)
+
+type ErrorBody struct {
+	Error string `json:"error"`
+}
+
+func writeJSON(w http.ResponseWriter, status int, payload any) {
+	w.Header().Set("Content-Type", "application/json; charset=utf-8")
+	w.WriteHeader(status)
+	if payload == nil {
+		_, _ = w.Write([]byte("null"))
+		return
+	}
+	// Go nil slices/maps encode as JSON null; clients expect [] / {}.
+	v := reflect.ValueOf(payload)
+	switch v.Kind() {
+	case reflect.Slice:
+		if v.IsNil() {
+			_, _ = w.Write([]byte("[]"))
+			return
+		}
+	case reflect.Map:
+		if v.IsNil() {
+			_, _ = w.Write([]byte("{}"))
+			return
+		}
+	}
+	_ = json.NewEncoder(w).Encode(payload)
+}
+
+func writeError(r *http.Request, w http.ResponseWriter, status int, message string, cause ...error) {
+	logAPIError(r, status, message, cause)
+	writeJSON(w, status, ErrorBody{Error: message})
+}
+
+func logAPIError(r *http.Request, status int, message string, cause []error) {
+	if status < 400 {
+		return
+	}
+	attrs := []any{
+		"module", "API",
+		"status", status,
+		"message", message,
+	}
+	if r != nil {
+		attrs = append(attrs, "method", r.Method, "path", r.URL.Path)
+	}
+	if len(cause) > 0 && cause[0] != nil {
+		attrs = append(attrs, "error", cause[0].Error())
+	}
+	switch {
+	case status >= 500:
+		slog.Error("request failed", attrs...)
+	default:
+		slog.Warn("request rejected", attrs...)
+	}
+}