fix(deps): install diffsynth from github (not on pypi)

The package 'diffsynth-studio' does not exist on pypi. The actual package
name in modelscope/DiffSynth-Studio's pyproject.toml is 'diffsynth' (v2.0.7),
and the only install path is from the git repo. HF Space build failed:

ERROR: Could not find a version that satisfies the requirement
diffsynth-studio>=0.5.0 (from versions: none)

Switch to: diffsynth @ git+https://github.com/modelscope/DiffSynth-Studio.git

requirements.txt

@@ -1,7 +1,7 @@
 # Core
 gradio==5.50.0
 spaces==0.30.0
-diffsynth-studio>=0.5.0
+diffsynth @ git+https://github.com/modelscope/DiffSynth-Studio.git
 torch>=2.4
 safetensors>=0.4.5
 huggingface-hub>=0.27