chore: update app.py, add app_bak.py, update requirements and autotune cache

Add GPU/CUDA diagnostics at startup

Update to torch2.8+cu128 environment and rename Render Mode

Fix device mismatch, use remote RMBG client, improve progress tracking, translate comments to English

refactor: use remote BRIA-RMBG-2.0 for background removal (no local GPU needed)

UI updates: Pixal3D V1.1 branding, camera settings, readme, ref thumbnail, GPU duration 240s, fix EXR path

Update title, add project page link, add autotune cache