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/repository/device_profile.rs

@@ -120,22 +120,34 @@ pub fn webview_can_decode_audio(codec: &str) -> bool {
 /// delegate this decision; it knows what its own renderer can decode and must
 /// apply that itself.
 ///
-/// The track that matters is the one the server will actually serve: the
-/// default, or the first when none is marked. An unknown codec is left alone —
-/// forcing a transcode on a guess would burn server CPU for files that play.
+/// The track that matters is the one the server will actually serve (see
+/// [`served_audio_codec`]). An unknown codec is left alone — forcing a transcode
+/// on a guess would burn server CPU for files that play.
 ///
 /// TRACES: UR-004 | DR-149 | UT-148
 pub fn audio_forces_transcode(streams: &[(Option<&str>, bool)]) -> bool {
-    let served = streams
+    match served_audio_codec(streams) {
+        Some(codec) => !webview_can_decode_audio(codec),
+        // No audio at all, or a codec the server did not name: leave it alone.
+        None => false,
+    }
+}
+
+/// The codec of the audio track the server will actually serve, given the
+/// source's audio streams as `(codec, is_default)` in source order: the default,
+/// or the first when none is marked.
+///
+/// `None` means "nothing to judge" — no audio streams, or the server named no
+/// codec for the one it would serve. Both callers of this rule treat that as
+/// leave-well-alone, never as a licence to assume compatibility.
+///
+/// TRACES: UR-004, UR-071 | DR-149, DR-171 | UT-148, UT-166
+pub fn served_audio_codec<'a>(streams: &[(Option<&'a str>, bool)]) -> Option<&'a str> {
+    streams
         .iter()
         .find(|(_, is_default)| *is_default)
-        .or_else(|| streams.first());
-
-    match served {
-        Some((Some(codec), _)) => !webview_can_decode_audio(codec),
-        // No audio at all, or a codec the server did not name: leave it alone.
-        Some((None, _)) | None => false,
-    }
+        .or_else(|| streams.first())
+        .and_then(|(codec, _)| *codec)
 }
 
 #[cfg(test)]
@@ -193,6 +205,25 @@ mod tests {
         assert!(!audio_forces_transcode(&[(None, true)]));
     }
 
+    /// The download path needs the codec itself, not just the verdict, so it can
+    /// tell the server what to re-encode. It picks the same track the streaming
+    /// verdict is formed from — one rule, one place.
+    ///
+    /// TRACES: UR-071 | DR-171 | UT-166
+    #[test]
+    fn the_served_codec_is_the_one_the_verdict_is_formed_from() {
+        assert_eq!(
+            served_audio_codec(&[(Some("aac"), false), (Some("eac3"), true)]),
+            Some("eac3")
+        );
+        assert_eq!(
+            served_audio_codec(&[(Some("eac3"), false), (Some("aac"), false)]),
+            Some("eac3")
+        );
+        assert_eq!(served_audio_codec(&[]), None);
+        assert_eq!(served_audio_codec(&[(None, true)]), None);
+    }
+
     #[test]
     fn a_dolby_device_does_not_advertise_dolby_for_video() {
         // The bug: a Motorola reporting c2.dolby.eac3.decoder direct-played