Fix Referer handling and improve media naming in downloads

[ja7ad]

A CDN with hotlink protection refuses an object to a client that does not say which page linked it. The extension has always captured the referer and sent it over the extbus; the app logged it and dropped it, so every request went out without one and the download failed where the browser succeeded — reported against a 4K MP4 that plays fine in the tab.

The referer now travels with the item, like StreamInfo::referer already
does for a manifest's segments: ExtDownload -> DownloadItem::referer ->
StartSpec -> request_headers, which the transfer and every probe share. probe_link takes those headers too, so the File Info dialog asks the same question the transfer will and reports a real size instead of ?.

Verified against the reporter's own signed URL: without the referer the probe gets nothing, with it 109566922 bytes.

Naming: download-stream already titled a captured stream after the page, download-url sent no name at all, so a direct file landed under whatever the CDN stored it as — a uuid, renamed by hand afterwards. A sniffed media file whose basename is opaque now takes the page title, as IDM does; a file that names itself, and any link the sniffer never saw, keep their own name.

Also surfaces the background-download switch in Options > Downloads. It was reachable only from inside the File Info dialog, which is why the same report asked for a confirm-before-downloading button that already existed.

Fixes #133

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.97%. Comparing base (1375044) to head (a08a667).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #134      +/-   ##
==========================================
- Coverage   73.98%   73.97%   -0.01%     
==========================================
  Files          69       69              
  Lines       29154    29154              
==========================================
- Hits        21569    21567       -2     
- Misses       7585     7587       +2     

see 2 files with indirect coverage changes

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