Save settings on the server and sync them to every device

Preferences lived only in the browser, so clearing it, using another device or the separate iOS
home-screen storage lost them. They are now saved in /data/settings.json (the audiobooks sign-in
encrypted), loaded at start-up, and existing browser-only values are uploaded once. A failed save
is reported instead of silent.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

Dockerfile

@@ -2,7 +2,7 @@ FROM node:22-alpine
 WORKDIR /app
 COPY package.json package-lock.json ./
 RUN npm ci --omit=dev
-COPY server.js auth.js cal.js ics.js caldav.js tasks.js ./
+COPY server.js auth.js cal.js ics.js caldav.js tasks.js settings.js ./
 COPY public ./public
 ENV PORT=8080 DATA_DIR=/data
 RUN mkdir -p /data && chown node:node /data