fix: pool-stashed transformer swap + MPS-safe vram + corrected model-zoo anchor

fix: post-review polish — slider defaults, error handling, comment

ci: ruff + pytest on push/pr (l1+l2, no gpu deps)

feat(backend): ZImageStudioBackend with @spaces.GPU and mode dispatch

feat(backend): zerogpu duration estimator (clamped 60-180s)