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>

frontend/src/pages/settings/SubscriptionGeneralTab.tsx

@@ -247,6 +247,26 @@ export default function SubscriptionGeneralTab({
                   onChange={onNumber((v) => updateSetting({ subUpdates: v }))}
                 />
               </SettingListItem>
+
+              <SettingListItem
+                paddings="small"
+                title={t('pages.settings.externalSubUserAgent')}
+                badge={
+                  <DefaultSettingTag
+                    settingKey="externalSubUserAgent"
+                    value={allSetting.externalSubUserAgent}
+                  />
+                }
+                description={t('pages.settings.externalSubUserAgentDesc')}
+              >
+                <Input
+                  value={allSetting.externalSubUserAgent}
+                  placeholder="v2rayNG/1.8.5"
+                  maxLength={512}
+                  allowClear
+                  onChange={(e) => updateSetting({ externalSubUserAgent: e.target.value })}
+                />
+              </SettingListItem>
             </>
           ),
         },