remember the library tab across restarts

The selected tab lived in rememberSaveable, which survives a rotation or a
process-death restore but not a cold start, so the app always came back on
Albums. It goes through DataStore now, like the per-tab sort and filters, which
were already durable - landing on a different tab than the one you had filtered
is what made the filters look reset.

app/src/test/kotlin/com/subtracks/data/prefs/UserPreferencesTest.kt

@@ -45,6 +45,22 @@ class UserPreferencesTest {
             file.delete()
         }
 
+    @Test
+    fun theSelectedLibraryTabRoundTrips() =
+        runTest {
+            val file = File.createTempFile("user-prefs", ".preferences_pb").apply { delete() }
+            val store = PreferenceDataStoreFactory.create(scope = backgroundScope) { file }
+            val prefs = UserPreferences(store)
+
+            assertEquals("Albums", prefs.libraryTab().first())
+
+            prefs.setLibraryTab("Playlists")
+
+            assertEquals("Playlists", prefs.libraryTab().first())
+
+            file.delete()
+        }
+
     @Test
     fun aListQueryStoredBeforeTheDownloadedFilterStillDecodes() =
         runTest {