Add matplotlib to requirements for the VRAM graphs

The two live graphs in app.py use matplotlib (Agg backend). It was already
a local transitive dep but not pinned in requirements.txt, so the HF Space
build failed with ModuleNotFoundError: No module named 'matplotlib'.

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

requirements.txt

@@ -1,4 +1,5 @@
 gradio==6.20.0
 huggingface_hub>=0.20.0
 requests>=2.28.0
-spaces>=0.26.0
+spaces>=0.26.0
+matplotlib>=3.8.0