Regenerates frontend contracts when backend source changes, in development.

Subscribes (via :file_system) to the host application's Elixir source and gettext directories. On a change it debounces briefly, reloads the host's code through Phoenix.CodeReloader (when an endpoint is configured and the reloader is running), and then runs PhoenixAssets.Generated.generate/1. The reload step matters: the file event fires on save, before anything recompiles the edited module -- without it, contracts would be generated from the previous code state and stay one edit behind. Because generation is content-gated, a change that does not affect any contract performs no write and triggers no Vite HMR; a change that does updates exactly the affected files, which Vite then hot reloads.

The watcher never watches the generated-output directory (it lives under the asset root, not under lib/), so it can never trigger itself -- there is no feedback loop.