Replace the job timeout message that hardcoded "background-removal" for
every tool with a tool-agnostic one that sets the CPU-vs-GPU expectation,
the usual reason heavy AI times out on modest hardware. The client-side
SSE stall message gets the same treatment. Both stay under friendlyError's
280-char limit so the guidance reaches the user instead of collapsing to a
generic "Processing failed".
Refs #591