Feat: blurred wallpaper variant and save-existing-theme flow

[tristonarmstrong]

This PR is meant as a seed planter — to start the conversation. I also looked at the existing blur functionality, and although it does blur, it's... not great... and doesn't really do what I wanted + its kinda hidden away. This is a single button, front and center, and offers a gorgeous background blur for those aesthetics nuts out there.

What

• Adds a heavy Gaussian blur wallpaper variant via BlurWallpaper — the blurred JPEG is applied as the desktop background while the original stays untouched for palette extraction
• Tracks OriginalWallpaperPath through the full theme pipeline so both the blurred and unblurred copies land in backgrounds/ (desktop cycler can switch between them)
• Reapplies the wallpaper explicitly after omarchy-theme-set to prevent it cycling to a stock image
• Adds ThemeFolderExists to let the frontend offer updating an existing saved theme in place

Why

Wallpapers with high-frequency detail (text, sharp edges, busy patterns) clash with overlaid UI elements. A blurred variant gives a clean backdrop while preserving palette extraction from the sharp source.

Testing

• TestBlurWallpaperCreatesVariant, TestBlurWallpaperRejectsNonImage, TestBlurWallpaperRejectsMissingFile
• TestPrepareThemeDirCopiesWallpaperVariants (blurred + original both copied)
• TestThemeFolderExists (omarchy + saved dirs, case normalization, invalid names)
• make test and npm run check pass

[tristonarmstrong]

damn out of date already.. i worked on this last night lmfao will sync

[tristonarmstrong]

theres lots of formatting changes.. ill work on undoing those - my b

[tristonarmstrong]

Please review, have your AI review. Let me know what you think, if its something you think is worth including.. I absolutely LOVE it but i can understand if its not something youere into

Feel free to commit straight to this if you have changes you wanna make or just let me know and i can do it

[bjarneo]

I misunderstood your post on X. Will have a look at this as soon as I have time!

[Chessing234]

blur pipeline and dual-wallpaper theme folders are well thought out.

[bjarneo]

The integration stores the source image and blur choice in blueprints, history, IPC state, and apply requests. Theme generation recreates the derived image and bundles both copies. Named saves use the native activation transaction. Wallpaper-only changes now preserve the active Omarchy theme. Verification passes: 109 frontend tests, Svelte checks, the frontend build, the native Go race suite, Go vet, and both CodeQL jobs.