Add debug flag and config init support

Introduce a `--debug` option and an `Init` subcommand to the CLI.
Add a new `config` module with loader and schema types for reading and
creating a default `veil.yaml` (supporting a local debug config path).
Update `.gitignore`, `default.nix`, and `flake.nix` comments to
reference
the templates directory.
Provide an example configuration template and integrate it into the
application’s entry point.

templates/example.veil.yaml

@@ -0,0 +1,14 @@
+apps:
+  - id: com.veil.jellyfin
+    name: jellyfin
+    title: Jellyfin
+    url: https://jellyfin.example.com
+    icon: https://jellyfin.example.com/favicon.ico
+    window:
+      width: 1280
+      height: 800
+      resizable: true
+      fullscreen: false
+      decorations: true
+      always_on_top: false
+      center: true