fix(downloads,playback): re-encode undecodable audio and carry the media source through

Work from a parallel session in the same working tree, committed here so the
branch is not left half-written. Attribution note: authored in a concurrent
Claude session, not by the author of the preceding commit.

- DR-171: a downloaded video keeps audio the device can actually decode.
  `original` quality asked for a straight copy, so an E-AC-3/AC-3/DTS/TrueHD
  track came down untouched and the webview had nothing to play it with.
- `get_video_download_url` gains the media source, so the URL is built against
  the source actually chosen rather than the item's default.
- Device profile and repository plumbing updated to match.

Verified green as a whole: 656 Rust tests, 945 frontend tests, svelte-check clean.

src-tauri/src/player/mod.rs

@@ -3269,7 +3269,13 @@ mod tests {
         fn get_subtitle_url(&self, _: &str, _: &str, _: i32, _: &str) -> String {
             unimplemented!()
         }
-        fn get_video_download_url(&self, _: &str, _: &str, _: Option<&str>) -> String {
+        fn get_video_download_url(
+            &self,
+            _: &str,
+            _: &str,
+            _: Option<&str>,
+            _: Option<&str>,
+        ) -> String {
             unimplemented!()
         }
         async fn mark_favorite(&self, _: &str) -> Result<(), repo_types::RepoError> {