library: fall back to the plural artist names for display

Keep the server's combined legacy artist/albumArtist string when it is
present, prefer the OpenSubsonic displayArtist when it is not, and otherwise
join the plural artist refs with Navidrome's separator. The normalized value
is stored in the display columns, so song rows, album cards, now playing,
menu subtitles, downloads, search and artist ordering all get it.

app/src/test/kotlin/com/subtracks/data/source/subsonic/SubsonicXmlTest.kt

@@ -240,6 +240,80 @@ class SubsonicXmlTest {
         )
     }
 
+    @Test
+    fun fallsBackToThePluralNamesWhenTheLegacyDisplayArtistIsBlank() {
+        val albums =
+            albums(
+                """
+                <subsonic-response status="ok">
+                  <albumList2>
+                    <album id="al1" name="Split" songCount="1">
+                      <artists id="ar1" name="A"/>
+                      <artists id="ar2" name="B"/>
+                    </album>
+                  </albumList2>
+                </subsonic-response>
+                """.trimIndent(),
+            )
+
+        assertEquals("A • B", albums.single().albumArtist)
+        assertEquals(listOf(ArtistRef("ar1", "A"), ArtistRef("ar2", "B")), albums.single().artists)
+    }
+
+    @Test
+    fun prefersTheOpenSubsonicDisplayArtistOverThePluralNames() {
+        val albums =
+            albums(
+                """
+                <subsonic-response status="ok">
+                  <albumList2>
+                    <album id="al1" name="Split" displayArtist="A feat. B" songCount="1">
+                      <artists id="ar1" name="A"/>
+                      <artists id="ar2" name="B"/>
+                    </album>
+                  </albumList2>
+                </subsonic-response>
+                """.trimIndent(),
+            )
+
+        assertEquals("A feat. B", albums.single().albumArtist)
+    }
+
+    @Test
+    fun fallsBackToThePluralSongArtistsWhenTheLegacyNameIsBlank() {
+        val songs =
+            songs(
+                """
+                <subsonic-response status="ok">
+                  <song id="s1" title="Collab" albumId="al1">
+                    <artists id="ar1" name="A"/>
+                    <artists id="ar2" name="B"/>
+                  </song>
+                </subsonic-response>
+                """.trimIndent(),
+            )
+
+        assertEquals("A • B", songs.single().artist)
+        assertEquals(listOf(ArtistRef("ar1", "A"), ArtistRef("ar2", "B")), songs.single().artists)
+    }
+
+    @Test
+    fun prefersTheOpenSubsonicDisplayArtistForSongs() {
+        val songs =
+            songs(
+                """
+                <subsonic-response status="ok">
+                  <song id="s1" title="Collab" albumId="al1" displayArtist="A feat. B">
+                    <artists id="ar1" name="A"/>
+                    <artists id="ar2" name="B"/>
+                  </song>
+                </subsonic-response>
+                """.trimIndent(),
+            )
+
+        assertEquals("A feat. B", songs.single().artist)
+    }
+
     @Test
     fun parsesTimestampsWithFractionsAndOffsets() {
         assertEquals(1789906657L, IsoDate.parse("2026-09-20T12:17:37.370827413Z"))