test: codify SPA PKCE flow stages

scripts/verify-pkce-flow-stages.sh

@@ -0,0 +1,21 @@
+#!/usr/bin/env sh
+set -eu
+
+npm --prefix frontend ci
+npm --prefix frontend test
+
+jq -e '
+  .clients[]
+  | select(.clientId == "spa-public")
+  | .publicClient == true
+    and .standardFlowEnabled == true
+    and .implicitFlowEnabled == false
+    and .directAccessGrantsEnabled == false
+    and .attributes["pkce.code.challenge.method"] == "S256"
+' keycloak/import/keycloak-patterns-realm.json >/dev/null
+
+rg -q 'response_type: "code"' frontend/src/app.js
+rg -q 'new InMemoryWebStorage' frontend/src/app.js
+rg -Fq 'cryptoApi.subtle.digest(' frontend/src/pkce.js
+rg -Fq '"SHA-256"' frontend/src/pkce.js
+echo "PKCE stages verified: verifier -> S256 challenge -> code -> verifier exchange"