[FEATURE] Wallhaven: Allow wallpaper preview

[jervw]

Submission checklist

• I have searched existing issues and confirmed this has not been requested before.
• I have checked existing pull requests for similar changes.

Feature type

New functionality

Feature summary

Add wallpaper preview functionality to the Wallhaven plugin before storing them in the wallpapers directory.

Motivation / use case

The plugin currently downloads and applies wallpapers directly to the wallpapers directory. Users who are selective about aesthetics, resolutions, or color palettes are forced to manually browse and delete unwanted wallpapers afterward. This is tedious, especially since identifying which wallpapers to remove requires opening files individually.

Proposed solution

Download wallpapers to a temporary directory first, allowing users to preview them before confirming they should be moved to the main wallpapers directory. This lets users test wallpapers without committing them.

Alternatives considered

No response

References / related projects

No response

Additional context

No response