fix: re-enable CI lint job and fix ~65 lint errors (partial)

Re-enable the golangci-lint job in CI (disabled Oct 2025), update to
Go 1.25 and golangci-lint-action v7. Fix errcheck, gosimple S1009,
staticcheck SA4031 and SA2001 errors across 54 files. Remaining ~39
lint errors will be addressed in follow-up commits.

server/api/endpoints.go

@@ -408,5 +408,5 @@ func (s *APIServer) ScreenShot(w http.ResponseWriter, r *http.Request) {
 	}
 	// Write the XML response with a 200 status code
 	w.WriteHeader(http.StatusOK)
-	w.Write(xmlData)
+	_, _ = w.Write(xmlData)
 }