Recursively locate wallpapers within subdirs of a wallpaper directory

### Feature Description

Allow the selection of a wallpaper directory, which may have sub-directories, and then recursively access wallpapers within all sub-directories within that selected directory.

### Use Case

I have wallpapers. Lots of wallpapers. Beautiful wallpapers.

They are all meticulously organised by theme or style into sub-directories within my ~/wallpapers dir. However, in DMS, I don't seem to be able to select that root, I can only select a wallpaper within one of the subdirs.

Thus, when I set it to auto cycle through wallpapers, it only does so within the single subdir.

It would be awesome if I could just select the root, and DMS is able to recursively locate and cycle through the wallpapers within.

### Compositor(s)

- [x] All compositors
- [ ] Niri
- [ ] Hyprland
- [ ] MangoWC (dwl)
- [ ] Sway
- [ ] Other (specify below)

### Proposed Solution

_No response_

### Alternatives/Existing Solutions

_No response_

No files, tests, or entry points are named in the issue. Locate the wallpaper-directory selection and automatic cycling entry points first; done means selecting a root directory discovers wallpapers in nested subdirectories and cycles through them across all compositors.

Written by the indexing model from the issue text.