Gate Cookbook "Run" on the model being downloaded
The What-Fits tab's quick "Run" button launched a serve task even when
the model was not downloaded. It POSTed directly to /api/model/serve and switched to the Running tab, so vLLM/SGLang would background-pull at launch (and llama.cpp just errors "No GGUF found") while the task showed as "running" without actually serving anything.
The Configure button and the Serve tab already gate on the cached-model
list; quick-Run did not. Mirror that gate: when the model isn't cached,
honor the button's "Download" half by kicking off the download instead of spawning a phantom serve task, and toast the user to Run again once it finishes.
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