fix: handle EACCES on data/settings.json in Docker containers

- settings-store: auto-detect writable path at startup — tries
  <cwd>/data/settings.json first, falls back to /tmp/lingvai-settings.json
  if the directory is not writable. Logs a warning when fallback is used.
  Also supports SETTINGS_PATH env var for explicit override.

- Dockerfile: switch from yarn to npm, explicitly create /app/data with
  chown nextjs:nodejs so the directory is writable at runtime without
  needing a privileged volume mount.

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

Dockerfile

@@ -3,31 +3,33 @@
 FROM node:lts-alpine AS deps
 RUN apk add --no-cache libc6-compat
 WORKDIR /app
-COPY package.json yarn.lock ./
-RUN yarn install --frozen-lockfile
+COPY package.json package-lock.json* ./
+RUN npm install --legacy-peer-deps
 
 FROM node:lts-alpine AS builder
 RUN apk add --no-cache curl
 WORKDIR /app
 
-RUN addgroup -g 1001 -S nodejs
-RUN adduser -S nextjs -u 1001
+RUN addgroup -g 1001 -S nodejs && \
+    adduser -S nextjs -u 1001
+
 COPY --chown=nextjs:nodejs . .
-COPY --from=deps /app/node_modules ./node_modules
-RUN chown nextjs:nodejs .
+COPY --from=deps --chown=nextjs:nodejs /app/node_modules ./node_modules
+
+# Ensure the data directory exists and is writable by the nextjs user
+RUN mkdir -p /app/data && chown -R nextjs:nodejs /app/data && chmod 755 /app/data
 
 USER nextjs
 
 EXPOSE 3000
 
 ENV NODE_ENV production
-
 ENV NEXT_TELEMETRY_DISABLED 1
 
 HEALTHCHECK --interval=1m --timeout=3s CMD curl -f http://localhost:3000/ || exit 1
 
-CMD NEXT_PUBLIC_SITE_DOMAIN=$site_domain\
+CMD NEXT_PUBLIC_SITE_DOMAIN=$site_domain \
     NEXT_PUBLIC_FORCE_DEFAULT_THEME=$force_default_theme \
     NEXT_PUBLIC_DEFAULT_SOURCE_LANG=$default_source_lang \
     NEXT_PUBLIC_DEFAULT_TARGET_LANG=$default_target_lang \
-    yarn build && yarn start
+    npm run build && npm start