web-app-chess: build/runtime hardening & feature enablement

Build: use Yarn 4 via Corepack; immutable install with inline builds.

Runtime: enable Corepack as user 'node', use project-local cache (/app/.yarn/cache), add curl; fix ownership.

Entrypoint: generate keys in correct dir; run 'yarn install --immutable --inline-builds' before migrations; wait for Postgres.

Config: enable matomo/css/desktop; notify 'docker compose build' on entrypoint changes.

Docs: rename README title to 'Chess'.

Ref: ChatGPT conversation (2025-09-03) — https://chatgpt.com/share/68b88126-7a6c-800f-acae-ae61ed577f46

roles/web-app-chess/config/main.yml

@@ -12,9 +12,9 @@ docker:
   volumes:
     data:           "chess_data"
 features:
-  matomo:           false
-  css:              false
-  desktop:          false
+  matomo:           true
+  css:              true
+  desktop:          true
   central_database: true
   logout:           false
   oidc:             false