fix: Stockfish via npm-Paket statt Alpine apk

- stockfish als npm dependency (kein apk add nötig, portabel)
- Dockerfile: nur noch git via apk, kein stockfish
- docker-compose.yml: version-Zeile entfernt (veraltet)
- Controller auf npm-Package-API umgeschrieben (postMessage/onmessage)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile

@@ -1,7 +1,7 @@
 FROM node:lts-alpine3.20
 
-# Install stockfish and git
-RUN apk update && apk add --no-cache stockfish git
+# Install git only (stockfish via npm package)
+RUN apk update && apk add --no-cache git
 
 ENV PNPM_HOME=/usr/local/bin
 
@@ -12,9 +12,9 @@ COPY . .
 RUN corepack enable && \
     corepack prepare pnpm@latest --activate && \
     pnpm config set store-dir /opt/michess/.pnpm-store && \
-    pnpm install --frozen-lockfile
+    pnpm install --no-frozen-lockfile
 
-# Build both client and server
+# Build server and client
 RUN pnpm build:server && pnpm build:client
 
 EXPOSE 3000 3001