Request: Protocol-specific shadowing

## Problem

I tried shadowing a folder so it isn't synced over WebDAV but this _also_ hides them from the Web UI.

## Describe the idea / solution you'd like

To be able to shadow a folder only for the WebDAV server, but not the Web UI.

There is also the more general problem of not being able to hide a folder. We can hide subdirectories but not directories in themselves.
e.g. I can put `h:` on it, but it will appear in the Web UI but not in WebDAV. I can mount a volume to `/var/empty` but then it is blank everywhere.

## Alternatives I've considered

I used the `h:` flag which worked as far as stopping sync, as did mounting to `/var/empty`, but they are all-or-nothing.

## Additional context

I have a WebDAV mount. Because it is the mount of [TiddlyWiki](https://tiddlywiki.com/), which has a dual single-file and node.js setup, it means I have to store things in a specific root structure

```
tiddlywiki
├── assets/
├── package.json
├── plugins/
├── single-file-snapshots/
├── tiddlers/
├── tiddlywiki.info
└── mywiki.html
```

Of these, only `mywiki.html` and the `assets` folder are necessary to sync, though I sync my `single-file-snapshots/` too. The `tiddlers/` folders have lots of files as do `plugins/`, and are strictly for node-wiki's use.

The structure must be maintained exactly like this so the relative paths inside my single-file wiki work.