Add Helm chart for Kubernetes deployment

charts/xslt-playground/README.md

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+# xslt-playground Helm Chart
+
+This chart deploys the frontend and backend of xslt-playground.
+
+## Configuration
+
+Key parameters in `values.yaml`:
+
+- `image.frontend` and `image.backend` – container images.
+- `firebaseSecretName` – name of the secret containing Firebase credentials.
+  - `firebase.credentialsKey` is mounted for the backend and referenced by
+    `GOOGLE_APPLICATION_CREDENTIALS`.
+  - `firebase.configKey` is used for the `VITE_FIREBASE_CONFIG` environment
+    variable of the frontend.
+- `databaseUrl` – connection string used by the backend.
+- `ingress` – configure ingress for the frontend service.
+- `hpa` – enable CPU-based autoscaling for both deployments.
+
+Create the secret before installing the chart:
+
+```bash
+kubectl create secret generic firebase-config \
+  --from-file=service-account.json=</path/to/serviceAccount.json> \
+  --from-file=firebase-config.json=</path/to/firebaseConfig.json>
+```
+
+Install the chart with:
+
+```bash
+helm install xslt charts/xslt-playground
+```
+