[Linuxuser67]

Summary

• Guard the torrent picker flow from the initial click through file selection and parsing.
• Use a drop guard so dismissal, read/parse errors, multi-file picker creation, cancellation, or normal completion all release the single-flight slot.
• Add a GTK-free regression test for overlapping-flow suppression and guard release.

Review

Independently checked the reported race against wire_torrent_picker: parsing now runs asynchronously while the button remains available, and the single-file enqueue could otherwise be scheduled more than once. The guard is acquired synchronously in the click callback, before the file dialog future is spawned, and is owned by that future.