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/docker-compose.yml.example

@@ -10,10 +10,6 @@ services:
       - CONFIG_PATH=/data/config.json
       - IMMICH_URL=http://your-immich-host:2283
       - IMMICH_API_KEY=your-immich-api-key-here
-      # Set this to your local timezone (e.g. America/New_York) if you use
-      # the "Quiet hours" setting -- without it, the container defaults to
-      # UTC, and quiet hours would run on UTC clock time instead of yours.
-      - TZ=UTC
       # Optional: gates the entire server -- the web UI (/, /api/*) AND
       # every device-facing /frame/* endpoint -- behind this shared secret.
       # Leave unset to keep it all open on a trusted LAN, same as before.