show album disc titles instead of generic disc numbers

Navidrome and gonic expose OpenSubsonic discTitles on the album response.
Parse them into a transient Album.discTitles map and persist each album's
titles in a child discs table (sourceId, albumId, disc, title) cascading
from albums. Songs keep only their disc number, and the album screen joins
by disc to render a title when one exists, falling back to "Disc N".

Titles are read opportunistically: a server that doesn't send discTitles
just leaves the table empty, so no API version negotiation is needed.
Titles apply to single- and multi-disc albums, are pruned when removed,
and are stored per album.

Schema 10 -> 11.

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

@@ -60,6 +60,28 @@ class SubsonicXmlTest {
         assertEquals(12L, albums[0].songCount)
     }
 
+    @Test
+    fun mapsAlbumDiscTitles() {
+        val document =
+            parse(
+                """
+                <subsonic-response status="ok">
+                  <albumList2>
+                    <album id="al1" name="The Wall" artist="Pink Floyd" artistId="ar1" songCount="26">
+                      <discTitles disc="1" title="The Calm"/>
+                      <discTitles disc="2" title="The Storm"/>
+                      <discTitles disc="3" title=""/>
+                    </album>
+                  </albumList2>
+                </subsonic-response>
+                """.trimIndent(),
+            )
+
+        val albums = SubsonicXml.albums(1, document)
+
+        assertEquals(mapOf(1L to "The Calm", 2L to "The Storm"), albums.single().discTitles)
+    }
+
     @Test
     fun parsesTimestampsWithFractionsAndOffsets() {
         assertEquals(1789906657L, IsoDate.parse("2026-09-20T12:17:37.370827413Z"))

app/src/test/kotlin/com/subtracks/data/sync/SyncServiceTest.kt

@@ -204,6 +204,102 @@ class SyncServiceTest {
             assertEquals(5678L, stored?.starred)
         }
 
+    @Test
+    fun syncStoresAlbumDiscTitles() =
+        runTest {
+            insertSource()
+            val source =
+                FakeMusicSource(
+                    albums = listOf(album("al1").copy(discTitles = mapOf(1L to "The Calm", 2L to "The Storm"))),
+                )
+
+            SyncService(db, source).sync()
+
+            assertEquals(mapOf(1L to "The Calm", 2L to "The Storm"), storedDiscs("al1"))
+        }
+
+    @Test
+    fun syncClearsDiscTitlesWhenTheServerStopsSendingThem() =
+        runTest {
+            insertSource()
+            val source =
+                FakeMusicSource(
+                    albums = listOf(album("al1").copy(discTitles = mapOf(1L to "The Calm"))),
+                )
+
+            SyncService(db, source).sync()
+            assertEquals(mapOf(1L to "The Calm"), storedDiscs("al1"))
+
+            source.albums = listOf(album("al1"))
+
+            SyncService(db, source).sync()
+            assertEquals(emptyMap<Long, String>(), storedDiscs("al1"))
+        }
+
+    @Test
+    fun discTitlesAreKeptPerAlbum() =
+        runTest {
+            insertSource()
+            val source =
+                FakeMusicSource(
+                    albums =
+                        listOf(
+                            album("al1").copy(discTitles = mapOf(1L to "First Album Disc")),
+                            album("al2"),
+                        ),
+                )
+
+            SyncService(db, source).sync()
+
+            assertEquals(mapOf(1L to "First Album Disc"), storedDiscs("al1"))
+            assertEquals(emptyMap<Long, String>(), storedDiscs("al2"))
+        }
+
+    @Test
+    fun removingAnAlbumRemovesItsDiscTitles() =
+        runTest {
+            insertSource()
+            val source =
+                FakeMusicSource(
+                    albums =
+                        listOf(
+                            album("al1").copy(discTitles = mapOf(1L to "The Calm")),
+                            album("al2").copy(discTitles = mapOf(1L to "The Storm")),
+                        ),
+                )
+
+            SyncService(db, source).sync()
+
+            source.albums = listOf(album("al2").copy(discTitles = mapOf(1L to "The Storm")))
+
+            SyncService(db, source).sync()
+
+            assertEquals(emptyMap<Long, String>(), storedDiscs("al1"))
+            assertEquals(mapOf(1L to "The Storm"), storedDiscs("al2"))
+        }
+
+    @Test
+    fun syncUpdatesADiscTitle() =
+        runTest {
+            insertSource()
+            val source = FakeMusicSource(albums = listOf(album("al1").copy(discTitles = mapOf(1L to "Old"))))
+
+            SyncService(db, source).sync()
+            assertEquals(mapOf(1L to "Old"), storedDiscs("al1"))
+
+            source.albums = listOf(album("al1").copy(discTitles = mapOf(1L to "New")))
+
+            SyncService(db, source).sync()
+            assertEquals(mapOf(1L to "New"), storedDiscs("al1"))
+        }
+
+    private suspend fun storedDiscs(albumId: String): Map<Long, String> =
+        db
+            .libraryDao()
+            .discs(1, albumId)
+            .first()
+            .associate { it.disc to it.title }
+
     private suspend fun <T : Any> PagingSource<Int, T>.allRows(): List<T> {
         val page = load(PagingSource.LoadParams.Refresh(key = null, loadSize = 100, placeholdersEnabled = false))
         return (page as PagingSource.LoadResult.Page).data

app/src/test/kotlin/com/subtracks/ui/AlbumDiscsTest.kt

@@ -8,6 +8,7 @@ import androidx.compose.ui.test.onAllNodesWithText
 import androidx.test.ext.junit.runners.AndroidJUnit4
 import com.github.takahirom.roborazzi.RobolectricDeviceQualifiers
 import com.subtracks.data.model.Album
+import com.subtracks.data.model.Disc
 import com.subtracks.data.model.Song
 import com.subtracks.ui.library.AlbumDetailScreen
 import com.subtracks.ui.theme.SubtracksTheme
@@ -32,6 +33,34 @@ class AlbumDiscsTest {
         assertEquals(1, nodesWithText("Disc 2"))
     }
 
+    @Test
+    fun discHeadersUseTheDiscTitleWhenPresent() {
+        setContent(
+            listOf(song("s-a", disc = 1, track = 1), song("s-b", disc = 2, track = 1)),
+            discs = listOf(disc(1, "The Calm"), disc(2, "The Storm")),
+        )
+        assertEquals(1, nodesWithText("The Calm"))
+        assertEquals(1, nodesWithText("The Storm"))
+        assertEquals(0, nodesWithText("Disc 1"))
+    }
+
+    @Test
+    fun discHeadersFallBackToTheDiscNumber() {
+        setContent(listOf(song("s-a", disc = 1, track = 1), song("s-b", disc = 2, track = 1)))
+        assertEquals(1, nodesWithText("Disc 1"))
+        assertEquals(1, nodesWithText("Disc 2"))
+    }
+
+    @Test
+    fun singleDiscAlbumShowsItsTitleWhenPresent() {
+        setContent(
+            listOf(song("s-a", disc = 1, track = 1), song("s-b", disc = 1, track = 2)),
+            discs = listOf(disc(1, "The Calm")),
+        )
+        assertEquals(1, nodesWithText("The Calm"))
+        assertEquals(0, nodesWithText("Disc 1"))
+    }
+
     @Test
     fun albumTracksShowTheirTrackNumbers() {
         setContent(listOf(song("s-a", disc = 1, track = 1), song("s-b", disc = 1, track = 2)))
@@ -51,12 +80,16 @@ class AlbumDiscsTest {
         assertEquals(0, nodesWithText("Disc 1"))
     }
 
-    private fun setContent(songs: List<Song>) {
+    private fun setContent(
+        songs: List<Song>,
+        discs: List<Disc> = emptyList(),
+    ) {
         composeRule.setContent {
             SubtracksTheme {
                 AlbumDetailScreen(
                     album = album(),
                     songs = songs,
+                    discs = discs,
                     coverArt = { _, _ -> null },
                     artwork = null,
                     onBack = {},
@@ -89,6 +122,11 @@ class AlbumDiscsTest {
         genre = null,
     )
 
+    private fun disc(
+        disc: Long,
+        title: String,
+    ) = Disc(sourceId = 1, albumId = "al-1", disc = disc, title = title)
+
     private fun album() =
         Album(
             sourceId = 1,