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.go

@@ -148,7 +148,7 @@ func doFetchSubscriptionLinks(rawURL string) ([]string, error) {
 		return nil, err
 	}
 	// Some providers gate the link body on a known client User-Agent.
-	req.Header.Set("User-Agent", "v2rayNG/1.8.5")
+	req.Header.Set("User-Agent", externalSubUserAgent())
 	// A 3x-ui donor with an HWID limit answers 404 when the header is empty (#6559).
 	if hwid := service.ExternalSubscriptionHwid(); hwid != "" {
 		req.Header.Set("X-HWID", hwid)
@@ -176,6 +176,19 @@ var (
 	errSubscriptionBodyTooLarge = &subError{"subscription response body exceeds size limit"}
 )
 
+// externalSubUserAgent returns the panel setting for external subscription
+// fetches, or the historical client UA when it is unset or the DB is unreachable.
+func externalSubUserAgent() string {
+	if database.GetDB() == nil {
+		return service.DefaultExternalSubUserAgent
+	}
+	ua, err := (&service.SettingService{}).GetExternalSubUserAgent()
+	if err != nil {
+		return service.DefaultExternalSubUserAgent
+	}
+	return ua
+}
+
 type subError struct{ msg string }
 
 func (e *subError) Error() string { return e.msg }