feat(sub): let the panel set the JSON subscription DNS servers (#6485)

* feat(sub): let the panel set the JSON subscription DNS servers

A baked routing profile (#6402) carries only the DNS its preset defines, so an
operator who wants their own resolvers has to override the whole profile or
patch the subscription behind a proxy.

Add the subJsonDns setting: either a full xray dns block or a bare array of
servers. It wins over the profile's DNS while leaving the profile's routing
rules intact, and reaches per-inbound, balancer and info-node documents alike.

The value is validated with xray's own schema (internal/xray/dnsconf): a block
the client could not load is rejected when the settings are saved and ignored
with a warning at request time, instead of being baked into every document.
Both the sub server and the settings API share that validator, so a stored
value can never be silently dropped.

xray's Build() is deliberately not used for validation: it resolves geosite
tokens from the geodata files and would reject valid configs whenever those
are absent from the panel's working directory.

* style(dnsconf): drop the ineffectual initial map assignment

golangci's ineffassign flagged the zero-value map whose value both paths
overwrite: the object branch now assigns the decoded map directly.

* docs(sub): scope the DNS setting to the documents it rewrites

The Routing header mirrored to Happ/INCY keeps the routing profile's own
resolvers, so the setting description and the header-source comment now say
so instead of claiming the profile's DNS is replaced everywhere.

Also trims two comments in the new dnsconf package to the repo's two-line cap.

internal/sub/sub.go

@@ -155,6 +155,11 @@ func (s *Server) initRouter() (*gin.Engine, error) {
 		SubJsonRoutingRules = ""
 	}
 
+	SubJsonDns, err := s.settingService.GetSubJsonDns()
+	if err != nil {
+		SubJsonDns = ""
+	}
+
 	SubJsonFinalMask, err := s.settingService.GetSubJsonFinalMask()
 	if err != nil {
 		SubJsonFinalMask = ""
@@ -316,6 +321,7 @@ func (s *Server) initRouter() (*gin.Engine, error) {
 		WithSUBJsonMux(SubJsonMux),
 		WithSUBJsonRules(SubJsonRules),
 		WithSUBJsonRoutingRules(SubJsonRoutingRules),
+		WithSUBJsonDns(SubJsonDns),
 		WithSUBJsonFinalMask(SubJsonFinalMask),
 		WithSUBJsonObservatory(SubJsonObservatory),
 		WithSUBClashEnableRouting(SubClashEnableRouting),