This commit introduces a new `DataNode` package, which provides an in-memory, `fs.FS`-compatible filesystem with a `debme`-like interface. The `DataNode` can be serialized to and from a TAR archive, making it suitable for storing downloaded assets.
The `pwa` and `serve` commands have been refactored to use the `DataNode`. The `pwa` command now packages downloaded PWA assets into a `DataNode` and saves it as a `.dat` file. The `serve` command loads a `.dat` file into a `DataNode` and serves its contents.
