feat(sub): auto-detect subscription format by User-Agent (Updated) (#5826)

* feat(settings): add subscription format controls

* feat(sub): auto-detect subscription formats

* fix(xray): validate balancer regexes before save

* Revert "fix(xray): validate balancer regexes before save"

This reverts commit 8a208ce71b.

* doc(endpoints): align indent spaces

* doc(settings): improve error message formatting in validateSubUserAgentRegex

- Use NewErrorf with proper formatting instead of NewError with string concatenation
- Add comment explaining the rationale for returning original pattern value
- This preserves the intentional design where empty input is stored as empty
  in the DB and inherited as the runtime default at read time

---------

Co-authored-by: Tomilla <5007859+Tomilla@users.noreply.github.com>
Co-authored-by: Sanaei <ho3ein.sanaei@gmail.com>

internal/web/controller/setting_test.go

@@ -0,0 +1,46 @@
+package controller
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/gin-gonic/gin"
+)
+
+func TestValidateRegex(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	router := gin.New()
+	NewSettingController(router.Group("/panel/api"))
+
+	tests := []struct {
+		name    string
+		body    string
+		success bool
+	}{
+		{name: "Go RE2 inline flag", body: `{"regex":"(?m)^general-purpose$"}`, success: true},
+		{name: "invalid expression", body: `{"regex":"["}`, success: false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			req := httptest.NewRequest(http.MethodPost, "/panel/api/setting/validateRegex", strings.NewReader(tt.body))
+			req.Header.Set("Content-Type", "application/json")
+			resp := httptest.NewRecorder()
+
+			router.ServeHTTP(resp, req)
+
+			if resp.Code != http.StatusOK {
+				t.Fatalf("status = %d, want 200; body=%s", resp.Code, resp.Body.String())
+			}
+			needle := `"success":true`
+			if !tt.success {
+				needle = `"success":false`
+			}
+			if !strings.Contains(resp.Body.String(), needle) {
+				t.Fatalf("body = %s, want %s", resp.Body.String(), needle)
+			}
+		})
+	}
+}