Make CUDA the primary Windows download

[wimi321]

Summary

• recommends the unified NVIDIA CUDA build for RTX 20/30/40/50 users
• removes the obsolete standalone RTX 50 download row
• keeps OpenCL, CPU, no-engine, and TensorRT choices clear for their intended users
• updates all seven download-page languages plus FAQ, setup guidance, and SEO copy
• adds a regression check so the old RTX 50 row and CPU-first recommendation do not return

Validation

• pnpm --dir website exec astro check
• pnpm --dir website build
• pnpm check:website
• desktop and 390px mobile browser snapshots
• git diff --check