feat(location): add offline mode and sun times caching (#22)

[prostopasta]

Closes #22

Summary

• Stores calculated sunrise/sunset times in ~/.cache/xfce-night-switch/sun_times upon successful NOAA calculation.
• When offline or if network/python calculation fails, automatically falls back to cached sun times, then to config DAY_START/DAY_END.
• Updates location preview in theme-settings.sh to read from cached times when offline.
• Adds automated test suite tests/test-offline-mode.sh covering NOAA calculation output format, cache persistence, and offline fallback.

[github-actions]

Test build ready -- PR #29

Install (Ubuntu/Debian/Mint):

wget -q 'https://github.com/prostopasta/xfce-night-switch/releases/download/pr-29-test-a2ffa2b/xfce-night-switch_1.6.0-pr.29.a2ffa2b_all.deb' -O /tmp/xns-test.deb \
  && sudo dpkg -i /tmp/xns-test.deb \
  && xfce-night-switch-setup

Checklist

• Panel icon visible after setup
• Click panel icon -- theme switches, icon updates
• No errors in journalctl --user -u xfce-night-switch-startup.service

After verifying all items, confirm by commenting /test-passed -- PR will auto-merge.

[prostopasta]

/test-passed