Fixes#4670
Separates the GGUF context slider ceiling from the currently active context length so lowering context via Chat Settings no longer locks the slider max to the reduced value.
- Backend: adds `max_context_length` to GGUF load/status responses, computed from the largest VRAM/KV-fit cap across all usable GPU subsets
- Frontend: stores `ggufMaxContextLength` and uses it for Context Length slider/input bounds; hydrates from both `/api/inference/load` and `/api/inference/status`
- Defaults UI ceiling to native context for CPU-only and fallback paths
- Seeds `effective_ctx` and `max_available_ctx` before GPU probing to prevent `UnboundLocalError` on probe failure
- Property fallback uses native `_context_length`, not effective `context_length`
2f0a5baa87