chore(build): bump Go toolchain to 1.27.0

Go 1.27.0 shipped on 2026-08-19. Raise the go directive and the builder
image so Docker and release builds pick it up; every CI job already reads
the version from go.mod, and golangci-lint v2.13.1 release binaries are
themselves built with go1.27.0, so the lint job needs no pin change.

internal/sub/remote_routing.go

@@ -370,8 +370,8 @@ func normalizeHappRouting(body []byte) (string, error) {
 
 	payload := ""
 	for _, prefix := range []string{"happ://routing/onadd/", "happ://routing/add/"} {
-		if strings.HasPrefix(text, prefix) {
-			payload = strings.TrimPrefix(text, prefix)
+		if after, ok := strings.CutPrefix(text, prefix); ok {
+			payload = after
 			break
 		}
 	}