smartplaylist: Change base directory for exported paths

### Problem

I run a Subsonic server on a different machine than the one I use for managing my music library. Both access the same music library, but they differ between machines (in fact, one is a Windows machine sharing a local path and one is Linux mounting that share). Subsonic requires playlists to have absolute paths as well, so I can't just use relative paths. I would like to be able to supply a playlist with a "from" and "to" mapping such that the written path has the "from" string replaced with the "to" string.

ex:
```
map_from: /mnt/music/
map_to: G:/Music/
```

would write `G:/Music/Artist/Album/01 - track.mp3` into the m3u for a file located at `/mnt/music/Artist/Album/01 - track.mp3`.

The issue names no files or tests. Start by locating the smartplaylist export path handling and its configuration entry point, then trace how exported playlist paths are constructed. Done means a configured from/to mapping rewrites the exported absolute paths as shown, while paths outside the mapping remain appropriately handled.

Written by the indexing model from the issue text.