fix(ci): stop requiring frontend/.yarn in docker build context

Keep .yarnrc.yml early for Yarn 4 config but remove COPY of frontend/.yarn, which may be absent in CI checkout and causes frontend image build failure.

Dockerfile.frontend

@@ -7,7 +7,6 @@ WORKDIR /app
 # Copy package files first for better caching
 COPY frontend/package*.json ./
 COPY frontend/.yarnrc.yml ./
-COPY frontend/.yarn/ ./.yarn/
 COPY frontend/yarn.lock* frontend/pnpm-lock.yaml* ./
 
 # Install dependencies