fix(docker): copy workspace package.json files before npm install

npm workspaces needs shared/package.json and server/package.json present
at install time to link @ttrpg/shared, otherwise the build stage fails
with "Module not found: Error: Can't resolve '@ttrpg/shared'".

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Dockerfile

@@ -7,8 +7,11 @@ LABEL stage="build-local-testing"
 
 WORKDIR /app
 
-# Copy package.json and package-lock.json (or yarn.lock)
+# Copy root package.json/lockfile plus workspace manifests so npm can
+# resolve the @ttrpg/shared and server workspace links during install.
 COPY package*.json ./
+COPY shared/package.json ./shared/
+COPY server/package.json ./server/
 
 # Install dependencies using the lock file for consistency
 RUN npm install