ui : share one download confirmation dialog across chips and rows
Extract DialogConfirmDownload for the destructive download actions (cancel an
in-flight download, delete a downloaded model), keyed by a DownloadConfirmAction
enum, and mount a single instance per surface instead of one per quant chip or
download row. The chips and download rows now only signal intent; the discover
options panel and the selector list own the dialog.
Assisted-by: llama-ui:Qwen3.8-Flash-Next