fix: Downgrade PyTorch to 2.2.0 for ZeroGPU compatibility

Issue:
- HF Spaces ZeroGPU error: "No candidate PyTorch version found"
- PyTorch 2.9.0 not supported by ZeroGPU infrastructure

Fix:
- Downgrade torch from 2.9.0 to 2.2.0
- ZeroGPU supports PyTorch versions 2.0.0-2.2.0

Technical Details:
- ZeroGPU uses shared NVIDIA H200 GPU infrastructure
- Only supports PyTorch 2.0.0, 2.1.0, and 2.2.0
- Local environments can still use any PyTorch version
- Comment added to clarify version constraint

Compatibility:
- ✅ ZeroGPU Spaces: torch 2.2.0 (compatible)
- ✅ CPU Upgrade Spaces: torch 2.2.0 (compatible)
- ✅ Local development: torch 2.2.0 (stable release)

References:
- https://huggingface.co/docs/hub/spaces-zerogpu
- ZeroGPU supported versions: PyTorch 2.0.0-2.2.0

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

requirements.txt

@@ -3,7 +3,7 @@ gradio==5.49.1
 
 # ML Core Libraries
 transformers==4.57.1
-torch==2.9.0
+torch==2.2.0  # ZeroGPU compatible (supports 2.0.0-2.2.0)
 safetensors==0.6.2
 accelerate==0.26.1