Server web UI: modern themeable layout with dark mode, timezone moved into config

Replaces the ad-hoc inline styling with a shared base template driven by
CSS custom properties (light/dark palettes), a card-based two-column
layout, and a persistent dark-mode toggle. Also moves quiet-hours'
timezone from the container's TZ env var into a proper web UI setting
(zoneinfo-backed), so it no longer needs a docker-compose.yml edit and
restart to change.

server/Dockerfile

@@ -2,10 +2,10 @@ FROM python:3.12-slim
 
 WORKDIR /app
 
-# tzdata: python:3.12-slim doesn't include it by default, so a TZ
-# environment variable (see docker-compose.yml.example -- used by the
-# "Quiet hours" setting) would silently fail to resolve and fall back to
-# UTC without this.
+# tzdata: python:3.12-slim doesn't include it by default, so the zoneinfo
+# database backing the web UI's "Timezone" setting (used by "Quiet hours")
+# would have no named zones to resolve without this -- ZoneInfo() would
+# raise for anything other than "UTC".
 RUN apt-get update && apt-get install -y --no-install-recommends tzdata \
     && rm -rf /var/lib/apt/lists/*