feat(ui): add canvas project save/load (.invk format)

[Pfannkuchensack]

Summary

Add ZIP-based .invk file format to save and restore the entire canvas state including all layers, masks, reference images, generation parameters, LoRAs, and embedded image files. Images are deduplicated on load — only missing images are re-uploaded from the project file.

• Always clear LoRAs on project load, even when project has none
• Fix jszip dependency ordering in package.json
• Add useAssertSingleton to SaveCanvasProjectDialog for consistency
• Add concurrency limit (max 5) for image fetch/upload requests
• Remove redundant deep-clone in remapCroppableImage (mutate in-place)
• Default project name to "Canvas Project" instead of empty string

Related Issues

#6984 #8466

QA Instructions

1. Open the canvas
2. Add layers, masks, reference images, set generation parameters and LoRAs
3. Save as .invk file via File → Save Canvas Project
4. Clear the canvas or restart the application
5. Load the .invk file via File → Load Canvas Project
6. Verify all layers, masks, reference images, parameters, and LoRAs are restored correctly
7. Load the same project again — verify already-existing images are not re-uploaded (deduplication)

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• ❗Changes to a redux slice have a corresponding migration
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)