Send the Jellyfin download token in the Authorization header

[pablofontanilla]

Summary

Refs #1598. Jellyfin audiobook downloads fail with HTTP 401.

createItemDownloadRequest sent the access token only as an ?api_key= query item. Our Jellyfin 12.1.0 server rejects that with a 401. This PR:

• sets Authorization: MediaBrowser Token="…" on the download request. It is set after wrapWithCustomHeaders, so a custom header cannot replace it (same rule as JellyfinHeaderInjector).
• removes the ?api_key= query item. This also keeps the token out of the download task's persisted taskDescription (SingleFileDownload-<url>).

No new files and no project.pbxproj changes. The tests are in the existing JellyfinQuickConnectTests.swift.

Header test on our 12.1.0 server

curl, same item, same API key:

Auth method	/Items/{id}/Download (range)	/Users/{uid}/Items/{id}
?api_key=	401	–
X-Emby-Token: <key>	401	401
X-MediaBrowser-Token: <key>	–	401
X-Emby-Authorization: MediaBrowser Token="<key>"	–	401
Authorization: MediaBrowser Token="<key>"	206	200

We did not check whether this is a 12.x default or a server setting.

Overlap with #1586

#1586 also fixes this download path, with X-Emby-Token and no ?api_key=. On our server, X-Emby-Token also gets a 401 (see the table), so #1586 may need the Authorization header too.

@GianniCarlo, you can pick the merge order. If this PR merges first, keep your version of createItemDownloadRequest when you rebase and change the header. If you prefer, I can rebase this PR onto #1586.

Out of scope

#1598 also says that Quick Connect does not work. This PR does not change Quick Connect.

Tests

• New JellyfinDownloadRequestTests (2 tests): the token is in the Authorization header, and the URL has no token and no api_key.
• Unit Tests plan (BookPlayerTests), iPhone 17 simulator: 535/535 pass.
• Manual test on the 12.1.0 server (iPhone 17 simulator): an 807 MB audiobook downloaded and imported. There was no 401. (Before the fix, the same request with ?api_key= returned 401 with curl.)

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