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/web/service/setting.go

@@ -36,6 +36,7 @@ var xrayTemplateConfig string
 
 const (
 	DefaultSubClashUserAgentRegex     = `(?i)(clash|mihomo)`
+	DefaultExternalSubUserAgent       = "v2rayNG/1.8.5"
 	DefaultSubJsonUserAgentRegex      = ``
 	DefaultRemarkTemplate             = "{{INBOUND}}-{{EMAIL}}|📊{{TRAFFIC_LEFT}}|⏳{{DAYS_LEFT}}D"
 	DefaultSubExpiredTemplate         = "⛔ {{EMAIL}} | Expired: {{EXPIRE_DATE}}"
@@ -146,6 +147,7 @@ var defaultValueMap = map[string]string{
 	"subDomain":                   "",
 	"subCertFile":                 "",
 	"subKeyFile":                  "",
+	"externalSubUserAgent":        DefaultExternalSubUserAgent,
 	"subUpdates":                  "12",
 	"subEncrypt":                  "true",
 	"subURI":                      "",
@@ -1052,6 +1054,17 @@ func (s *SettingService) GetSubUpdates() (string, error) {
 	return s.getString("subUpdates")
 }
 
+func (s *SettingService) GetExternalSubUserAgent() (string, error) {
+	value, err := s.getString("externalSubUserAgent")
+	if err != nil {
+		return DefaultExternalSubUserAgent, err
+	}
+	if value = strings.TrimSpace(value); value == "" {
+		return DefaultExternalSubUserAgent, nil
+	}
+	return value, nil
+}
+
 func (s *SettingService) GetSubEncrypt() (bool, error) {
 	return s.getBool("subEncrypt")
 }