upgrade CUDA backend from cu126 to cu128 and fix GPU settings UI

Upgrade CUDA toolkit from 12.6 (cu126) to 12.8 (cu128) for proper
RTX 50-series (Blackwell) GPU support. Users with RTX 5070/5080/5090
were reporting CUDA detection failures with cu126.

Also fix the GPU Acceleration settings panel where the 'Switch to CPU
Backend' button was unreachable — it was inside a conditional block
that required !isCurrentlyCuda, making it impossible to switch back
to CPU once running on CUDA.

Closes #315

backend/build_binary.py

@@ -370,7 +370,7 @@ def build_server(cuda=False):
                     "torchvision",
                     "torchaudio",
                     "--index-url",
-                    "https://download.pytorch.org/whl/cu126",
+                    "https://download.pytorch.org/whl/cu128",
                     "--force-reinstall",
                     "-q",
                 ],

backend/requirements.txt

@@ -8,7 +8,7 @@ sqlalchemy>=2.0.0
 alembic>=1.13.0
 
 # ML models
-torch>=2.1.0
+torch>=2.7.0
 transformers>=4.36.0,<=4.57.6
 accelerate>=0.26.0
 huggingface_hub>=0.20.0

backend/services/cuda.py

@@ -32,7 +32,7 @@ PROGRESS_KEY = "cuda-backend"
 
 # The current expected CUDA libs version.  Bump this when we change the
 # CUDA toolkit version or torch's CUDA dependency changes (e.g. cu126 -> cu128).
-CUDA_LIBS_VERSION = "cu126-v1"
+CUDA_LIBS_VERSION = "cu128-v1"
 
 
 def get_backends_dir() -> Path: