Per-model (model config modal): the "Engine / card" selector gains an "All GPUs
(split across every backend)" option. Choosing it sets gpu_split and reveals a
"Weight distribution" field (tensor_split, e.g. 0.8,0.2); blank = auto. A real
engine name still pins as before.

Global (Settings): "Split models across all GPUs" checkbox + default weight
distribution field, persisted to offload.gpu_split / offload.tensor_split and
pushed to live global_args. Per-model "All GPUs" overrides the global default.

api: settings GET/POST now expose+accept offload.gpu_split / tensor_split.

backend: when gpu_split is on and no ratio is given, auto-derive tensor_split
proportional to each device's FREE VRAM (new _per_device_free_vram_gb, llama.cpp
device order: CUDA first, then Vulkan/AMD) — e.g. 24 GB 3090 + 8 GB RX 580 ->
~0.75/0.25 — so the bigger card carries more without the user computing it.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>