Desktop plugins reach the backend only through ws JSON-RPC; image
generation existed solely as a model tool, so plugin UI (avatar pickers,
artifact panes) could not generate images. image.generate delegates to
the configured image_generate backend, supports probe:true for cheap
availability checks, and returns the result as both the backend ref and
a size-capped data URL — remote-gateway clients cannot read gateway-host
file paths and hosted URLs are often CORS-opaque, so the data URL works
identically over local and remote gateways. Pool-dispatched; missing
backend degrades to a soft {available:false} instead of an RPC error.
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