fix(dirvish): guard nil file and reject path-traversal playlist names

cj/set-wallpaper passed `(dired-file-name-at-point)` straight to `expand-file-name`, so running it with no file at point raised a bare `wrong-type-argument` instead of a clear error. cj/dired-create-playlist-from-marked expanded the raw playlist name under `music-dir` without checking it, so a name like "../foo" or "/etc/foo" would write outside the music directory.

I added a nil-file guard to set-wallpaper and a `cj/--playlist-name-safe-p` check that rejects any name carrying a directory separator before the path is built. Both paths now fail cleanly with a user-error. Regression tests went into the existing wrapper and playlist test files.

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