feat: add wallpaper post-hook for dynamic theming (#61)

* feat: add wallpaper post-hook for dynamic theming

Adds support for running custom shell commands after wallpaper changes
via the `wallpaper.postHook` config option in `~/.config/caelestia/cli.json`.

The wallpaper path is made available to the hook via the $WALLPAPER_PATH
environment variable. This enables integration with tools like `matugen`
or `pywal` for dynamic theming based on wallpaper colors.

The hook runs after apply_colours() in set_wallpaper(), ensuring it
executes for all wallpaper change methods (UI, IPC, and direct CLI).

* Replaced comment in example config with no-op command

README.md

@@ -133,6 +133,9 @@ All configuration options are in `~/.config/caelestia/cli.json`.
     "record": {
         "extraArgs": []
     },
+    "wallpaper": {
+        "postHook": "echo $WALLPAPER_PATH"  
+    },
     "theme": {
         "enableTerm": true,
         "enableHypr": true,