fix: allow duplicate downloads by unique filenames

[sinkaroid]

Proposed changes

• Prevent 'false success' where repeated downloads looks no action in Gallery due to same file path.
• Ensure re-downloading older images (e.g., from last year) creates a new, visible file instead of silently reusing the same path with no action.

What changed

• Allow duplicate images for redownloading
• Added getUniqueFilePath() to append _1, _2, ... until a free filename is found.

Real use case

I love the simplicity for PxViewr, I hope this project still maintained..
So.. I often re-download older images saved long ago (e.g., last year, last month or even last week). I expected the app to overwrite or at least surface a new file, but because the same path is reused, Gallery shows no visible changes.

This change ensures a new filename for duplicates, so re-downloading old images actually produces a visible new file.