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/web/entity/entity.go

@@ -108,6 +108,7 @@ type AllSetting struct {
 	SubJsonMux                  string `json:"subJsonMux" form:"subJsonMux"`
 	SubJsonRules                string `json:"subJsonRules" form:"subJsonRules"`
 	SubJsonRoutingRules         string `json:"subJsonRoutingRules" form:"subJsonRoutingRules"`
+	SubJsonDns                  string `json:"subJsonDns" form:"subJsonDns"`
 	SubJsonFinalMask            string `json:"subJsonFinalMask" form:"subJsonFinalMask"`
 	SubJsonObservatory          string `json:"subJsonObservatory" form:"subJsonObservatory"`
 	SubThemeDir                 string `json:"subThemeDir" form:"subThemeDir"`