feat(sub): make external subscription fetch User-Agent configurable (#6613)

* feat(sub): make external subscription fetch User-Agent configurable

Some providers reject fetches that do not send a known client User-Agent.
Expose externalSubUserAgent as a panel setting (default v2rayNG/1.8.5)
and use it when fetching client external subscription URLs.

Fixes #6383

* ci: retrigger frontend after npm registry maintenance

The frontend job failed solely on `npm audit` while registry.npmjs.org
returned 503 (Service Under Maintenance). Lint, typecheck, vitest, vite
build, and storybook all passed. Local `npm audit --omit=dev
--audit-level=high` now reports 0 vulnerabilities.

* fix(sub): fall back to the default UA when the DB is not initialised

externalSubUserAgent read the setting through SettingService.getSetting,
which calls Model() on database.GetDB() and panics on a nil *gorm.DB.
The fetch path's other DB read, service.ExternalSubscriptionHwid, already
treats a nil DB as unreachable and sends no header; the new UA lookup
did not, so any fetch before InitDB panicked instead of sending the
historical v2rayNG/1.8.5.

Production initialises the DB before the sub server starts, but the
internal/sub fetch tests run without one: under make test-go's
-shuffle=on, whenever one of them ran before the first InitDB test the
panic aborted the whole package. Reproduced deterministically with
go test -run '^TestDoFetchSubscriptionLinks_RejectsOversizedBody$'.

---------

Co-authored-by: mrchatam <mrchatam@users.noreply.github.com>
Co-authored-by: MHSanaei <ho3ein.sanaei@gmail.com>

internal/sub/external_subscription_test.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"net/http"
 	"net/http/httptest"
+	"path/filepath"
 	"strconv"
 	"strings"
 	"sync"
@@ -298,3 +299,57 @@ func TestExpandEntryCacheHitWritesNothing(t *testing.T) {
 		t.Fatalf("cache hit wrote fetch status: %#v", after)
 	}
 }
+
+func TestFetchUsesConfiguredExternalSubUserAgent(t *testing.T) {
+	resetSubscriptionCache(t)
+	if err := database.InitDB(filepath.Join(t.TempDir(), "ua.db")); err != nil {
+		t.Fatalf("InitDB: %v", err)
+	}
+	t.Cleanup(func() { _ = database.CloseDB() })
+
+	const customUA = "Happ/4.2.1"
+	if err := database.GetDB().Create(&model.Setting{
+		Key:   "externalSubUserAgent",
+		Value: customUA,
+	}).Error; err != nil {
+		t.Fatalf("save setting: %v", err)
+	}
+
+	var gotUA string
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		gotUA = r.Header.Get("User-Agent")
+		_, _ = w.Write([]byte("vless://uuid@host:443?security=none#x"))
+	}))
+	defer srv.Close()
+
+	res := fetchSubscriptionLinks(srv.URL)
+	if res.err != nil {
+		t.Fatalf("fetch: %v", res.err)
+	}
+	if gotUA != customUA {
+		t.Fatalf("User-Agent = %q, want %q", gotUA, customUA)
+	}
+}
+
+func TestFetchFallsBackToDefaultExternalSubUserAgent(t *testing.T) {
+	resetSubscriptionCache(t)
+	if err := database.InitDB(filepath.Join(t.TempDir(), "ua-default.db")); err != nil {
+		t.Fatalf("InitDB: %v", err)
+	}
+	t.Cleanup(func() { _ = database.CloseDB() })
+
+	var gotUA string
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		gotUA = r.Header.Get("User-Agent")
+		_, _ = w.Write([]byte("vless://uuid@host:443?security=none#x"))
+	}))
+	defer srv.Close()
+
+	res := fetchSubscriptionLinks(srv.URL)
+	if res.err != nil {
+		t.Fatalf("fetch: %v", res.err)
+	}
+	if gotUA != "v2rayNG/1.8.5" {
+		t.Fatalf("User-Agent = %q, want default v2rayNG/1.8.5", gotUA)
+	}
+}