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/translation/zh-TW.json

@@ -1273,6 +1273,8 @@
       "subDomainDesc": "訂閱服務監聽的域名（留空表示監聽所有域名和 IP）。當「反向代理 URI」為空時，也會作為顯示的訂閱連結的備援域名——如果面板和訂閱透過不同的域名存取(例如位於反向代理之後)，請設定「反向代理 URI」。",
       "subUpdates": "更新間隔",
       "subUpdatesDesc": "客戶端應用中訂閱 URL 的更新間隔（單位：小時）",
+      "externalSubUserAgent": "外部訂閱 User-Agent",
+      "externalSubUserAgentDesc": "面板擷取用戶外部訂閱 URL 時傳送的 User-Agent。留空則使用預設值（v2rayNG/1.8.5）。部分服務商會拒絕未知用戶端。",
       "subEncrypt": "編碼",
       "subEncryptDesc": "訂閱服務返回的內容將採用 Base64 編碼",
       "subURI": "反向代理 URI",