feat(nix): add custom background to WezTerm

- Configured a new custom background image for the WezTerm terminal.
- Added brightness, hue, and saturation adjustments for better visual customization.
- Included the background image file in the Nix configuration.

homes/x86_64-linux/harald@amd/default.nix

@@ -79,6 +79,12 @@
 
     config.enable_kitty_keyboard = true
     config.enable_scroll_bar = true
+    config.window_background_image = '${./terminal-background.png}'
+    config.window_background_image_hsb = {
+      brightness = 0.3,
+      hue = 1.0,
+      saturation = 1.0,
+    }
 
     config.term = 'wezterm'