`ark-cli`: Server mode and cache pre-generation

Would be cool to have a subcommand `serve` which monitors list of roots and generates various cache for every root. This would help low-power devices to obtain the cache through the network instead of generating it on the device. As soon as new file synced to the server instance, `ark-cli` would generate preview, thumbnail and maybe some another kind of cache in the future.

The cache is replicated and persistent, meaning, it is wired through Syncthing to other devices (later we will implement our own syncing mechanism).

Start at the ark-cli command entry point and trace how roots and existing cache generation are handled. Define the serve subcommand's monitoring, preview and thumbnail generation, persistence, and Syncthing behavior before implementation. Done means the server can monitor configured roots and generate persistent cache for newly synced files.

Written by the indexing model from the issue text.