Rewrite i3lock lockscreen with wallpaper, cover resize and caching

- Replace screenshot+blur with static wallpaper (void-minimalist.png)
- Resize to display resolution with cover mode (resize^ + extent)
- Cache resized image in ~/.cache/i3lock/ to avoid re-encoding
- Invalidate cache when wallpaper or resolution changes
- Add xdotool to void_packages_base for display geometry detection

ansible/inventory/group_vars/void.yml

@@ -35,6 +35,7 @@ void_packages_base:
   - socklog-void
   - tmux
   - ufw
+  - xdotool
   - void-repo-nonfree
   - vpm
   - vsv