Add timezone selector to quiet hours in web UI

[jampick]

Summary

• The quiet hours section in the web UI previously showed the Pi's system timezone as read-only info text, with no way to change it
• Users can now pick any IANA timezone from a dropdown (e.g. America/New_York, Europe/London, Asia/Tokyo) so quiet hours work correctly regardless of what timezone the Pi is set to
• Falls back to system local time when left at "System default"

Changes

• pi/appliance.py — add quiet_tz key to default config ("" = system timezone)
• pi/webapp/server.py — add SELECTABLE_TIMEZONES list with ~40 common IANA zones; validate submitted quiet_tz; pass timezone list to template
• pi/webapp/templates/index.html — replace static timezone label with a <select> dropdown populated from the server
• printpulse/watch.py — _is_in_quiet_hours() accepts optional tz (IANA string) and uses zoneinfo.ZoneInfo; run_watch_loop() accepts quiet_tz
• printpulse/app.py — add --quiet-tz CLI arg
• printpulse/pi_launcher.py — pass --quiet-tz to watch loop when set in config
• tests/test_watch.py — add TestIsInQuietHoursWithTimezone covering explicit TZ and invalid TZ fallback

Test plan

• Run python -m pytest tests/ -v -s — all tests pass
• On the Pi: open web UI, enable quiet hours, confirm timezone dropdown appears with "System default" pre-selected
• Select a different timezone (e.g. America/New_York), save — confirm the watcher log shows the chosen TZ in the quiet hours label
• Leave timezone at "System default" — confirm existing behaviour is unchanged

🤖 Generated with Claude Code