common : allow --offline in llama download (#25091)

Expose the existing --offline flag to llama download so a script can run it to check whether a model is already cached and ready to be served without touching the network.

Also fix a latent use-after-free in the URL-task on_done callback: first_path is block-scoped and was captured by reference, but invoked after the block ends.

Signed-off-by: Adrien Gallouët [email protected]