#444 macOS: Quick Look previews for downloads

[Anonymous]

Originally created by: kristofferR

Problem

Downloads open in another app or show in Finder. The planned Downloads view (#252) would follow the same pattern. On macOS, Quick Look is the expected way to glance at a file without leaving the app.

Goal

Preview downloaded Messenger attachments with Quick Look inside Chatpane.

Scope

• Add a Quick Look action, opened with Space or a button, to downloads in the [#252] Downloads view and to the download-completed toast where it fits.
• Use QLPreviewPanel with a native data source that returns the file's NSURL. Resolve the opaque download ID natively, as [#252] requires. No filesystem paths pass through the page.
• Only files that pass the download policy and still exist can be previewed. Moved or deleted files fail cleanly.
• macOS only. Windows and Linux keep Open and Show in folder.

Verification

• Preview an image, a video, a PDF and an Office document. Space and Escape open and close the panel, and the arrow keys move between items when there are several.
• A stale ID or a deleted file shows an error and never opens the panel.

Depends on [#252].