feat(sub): per-client external links and remote subscriptions

Add a Links tab to the client form for attaching third-party share
links and remote subscription URLs per client. They are merged into
the client's raw/JSON/Clash subscription output: links are emitted
verbatim and parsed for JSON/Clash; subscription URLs are fetched
(cached, with a short timeout) and their configs merged in.

i18n keys added across all 13 locales.

internal/sub/clash_service.go

@@ -25,9 +25,16 @@ func (s *SubClashService) GetClash(subId string, host string) (string, string, e
 	// Set per-request state so resolveInboundAddress sees the node map.
 	s.SubService.PrepareForRequest(host)
 	inbounds, err := s.SubService.getInboundsBySubId(subId)
-	if err != nil || len(inbounds) == 0 {
+	if err != nil {
 		return "", "", err
 	}
+	externalLinks, err := s.SubService.getClientExternalLinksBySubId(subId)
+	if err != nil {
+		return "", "", err
+	}
+	if len(inbounds) == 0 && len(externalLinks) == 0 {
+		return "", "", nil
+	}
 
 	var proxies []map[string]any
 
@@ -43,6 +50,18 @@ func (s *SubClashService) GetClash(subId string, host string) (string, string, e
 			proxies = append(proxies, s.getProxies(inbound, client, host)...)
 		}
 	}
+	for _, ext := range externalLinks {
+		for _, el := range expandEntry(ext) {
+			name := el.Name
+			if name == "" {
+				name = ext.Email
+			}
+			if proxy := s.clashProxyFromExternal(el.Link, name); proxy != nil {
+				seenEmails[ext.Email] = struct{}{}
+				proxies = append(proxies, proxy)
+			}
+		}
+	}
 
 	if len(proxies) == 0 {
 		return "", "", nil