Add DoneAt to the download task model: set when a task finishes and
cleared when it is downloaded again. The value flows through the API
model and task record, and is shown as "Completed at" in the task
drawer info tab next to the creation time.
- Space the update chip off the card title to match the detail header
- Clear the extension's update flag right after a successful upgrade so
the chip disappears without waiting for the background re-check
- Use the concise Update label across all locales
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