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/json_dns.go

@@ -0,0 +1,20 @@
+package sub
+
+import (
+	"github.com/mhsanaei/3x-ui/v3/internal/logger"
+	"github.com/mhsanaei/3x-ui/v3/internal/xray/dnsconf"
+)
+
+// The subJsonDns setting holds either a full xray dns block or a bare array of
+// servers; dnsconf validates it against the parser the client runs.
+
+// SetDnsConfig overrides the dns block of every emitted document; an unusable
+// value is logged and ignored so a typo cannot take subscriptions down.
+func (s *SubJsonService) SetDnsConfig(raw string) {
+	block, err := dnsconf.Parse(raw)
+	if err != nil {
+		logger.Warningf("subJsonDns: %v; keeping the template DNS", err)
+		return
+	}
+	s.dnsBlock = block
+}