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>

docs/public/openapi.json

@@ -69,6 +69,9 @@
             "minimum": 0,
             "type": "integer"
           },
+          "externalSubUserAgent": {
+            "type": "string"
+          },
           "externalTrafficInformEnable": {
             "type": "boolean"
           },
@@ -522,6 +525,7 @@
           "discordMemory",
           "discordRunTime",
           "expireDiff",
+          "externalSubUserAgent",
           "externalTrafficInformEnable",
           "externalTrafficInformURI",
           "happLinkEnable",
@@ -704,6 +708,9 @@
             "minimum": 0,
             "type": "integer"
           },
+          "externalSubUserAgent": {
+            "type": "string"
+          },
           "externalTrafficInformEnable": {
             "type": "boolean"
           },
@@ -1181,6 +1188,7 @@
           "discordMemory",
           "discordRunTime",
           "expireDiff",
+          "externalSubUserAgent",
           "externalTrafficInformEnable",
           "externalTrafficInformURI",
           "happLinkEnable",