Introduce SSRF-safe HTTP transport for custom geo operations by adding ssrfSafeTransport and isBlockedIP helpers. The transport resolves hosts and blocks loopback, private, link-local and unspecified addresses, returning ErrCustomGeoSSRFBlocked on violations. Update probeCustomGeoURLWithGET, probeCustomGeoURL and downloadToPathOnce to use the safe transport. Also add the new error ErrCustomGeoSSRFBlocked and necessary imports. Minor whitespace/formatting adjustments in subClashService.go, web/entity/entity.go and web/service/setting.go.
