Add bun integration tests for docs flow, update integration guide

- Add test/integration.test.ts: getServicePublicKey, full flow (register, login, createCollection, createPointFeature, listFeatures)
- Update docs example: registerBySigningServiceKey then loginWithSignature
- Document integration tests in typescript-frontend-integration.md

Made-with: Cursor

internal/http/handlers.go

@@ -103,7 +103,12 @@ func statusFromErr(err error) int {
 }
 
 func writeErr(w http.ResponseWriter, err error) {
-	writeJSON(w, statusFromErr(err), map[string]string{"error": err.Error()})
+	payload := map[string]string{"error": err.Error()}
+	if errors.Is(err, app.ErrRegistrationNotConfigured) {
+		payload["code"] = "REGISTRATION_NOT_CONFIGURED"
+		payload["hint"] = "Run ./bin/gen-server-keys.sh to create etc/server-service.env, or set ADMIN_PUBLIC_KEY before starting the API."
+	}
+	writeJSON(w, statusFromErr(err), payload)
 }
 
 func bearerToken(r *http.Request) (string, error) {