fix(dimming): direct test trigger, setup symlink, and xrandr auto-fallback

[prostopasta]

Summary

• Symlink monitor-dimming.sh in xfce-night-switch-setup.
• Directly trigger monitor-dimming.sh "$cur_mode" in show_dimming_dialog on 'Test now' and 'OK' so changes apply immediately even if current theme is already active.
• Auto-fallback to xrandr if ddcutil is unavailable or lacks I2C device permissions.

[github-actions]

Test build ready -- PR #30

Install (Ubuntu/Debian/Mint):

wget -q 'https://github.com/prostopasta/xfce-night-switch/releases/download/pr-30-test-75c7e16/xfce-night-switch_1.7.0-pr.30.75c7e16_all.deb' -O /tmp/xns-test.deb \
  && sudo dpkg -i /tmp/xns-test.deb \
  && xfce-night-switch-setup

Checklist

• Panel icon visible after setup
• Fresh install: setup runs once, no duplicate output
• 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