Two fixes for GGUF variant dropdown:
1. useGpuInfo now sums memory across all GPU devices instead of only
reading devices[0]. This matches llama-server's multi-GPU allocation
where models can be split across GPUs.
2. When the backend-recommended variant (e.g. UD-Q4_K_XL) exceeds total
GPU VRAM, the frontend picks the largest variant that fits instead.
If all variants are OOM, it recommends the smallest one (most likely
to work with --fit).
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