Several small fixes (#1612)

## Description
- Fixes showing "null" in the subtitle for some tv episodes (usually
daily shows like Jeopardy)
- Partially fix holding back to return to the previous grid position
(still has focus issues)
- Show/hide tabs on the discover page similar to other pages in the app
- Prevent a warning server-side if a negative playback timestamp is sent

### Related issues
N/A

### Testing
Emulator

## Screenshots
N/A

## AI or LLM usage
None

app/src/main/java/com/github/damontecres/wholphin/data/model/BaseItem.kt

@@ -12,6 +12,7 @@ import com.github.damontecres.wholphin.ui.detail.series.SeasonEpisodeIds
 import com.github.damontecres.wholphin.ui.dot
 import com.github.damontecres.wholphin.ui.formatDateTime
 import com.github.damontecres.wholphin.ui.getDateFormatter
+import com.github.damontecres.wholphin.ui.joinNotBlank
 import com.github.damontecres.wholphin.ui.nav.Destination
 import com.github.damontecres.wholphin.ui.playback.playable
 import com.github.damontecres.wholphin.ui.roundMinutes
@@ -57,14 +58,14 @@ data class BaseItem(
 
     val title get() = if (type == BaseItemKind.EPISODE) data.seriesName else name
 
-    val subtitle
-        get() =
-            when (type) {
-                BaseItemKind.EPISODE -> data.seasonEpisode + " - " + name
-                BaseItemKind.SERIES -> data.seriesProductionYears
-                BaseItemKind.AUDIO -> listOfNotNull(data.album, artistsString).joinToString(" - ")
-                else -> data.productionYear?.toString()
-            }
+    val subtitle: String? by lazy {
+        when (type) {
+            BaseItemKind.EPISODE -> listOf(data.seasonEpisode, name).joinNotBlank(" - ")
+            BaseItemKind.SERIES -> data.seriesProductionYears
+            BaseItemKind.AUDIO -> listOf(data.album, artistsString).joinNotBlank(" - ")
+            else -> data.productionYear?.toString()
+        }
+    }
 
     val subtitleLong: String? by lazy {
         if (type == BaseItemKind.EPISODE) {
@@ -72,7 +73,7 @@ data class BaseItem(
                 add(data.seasonEpisodePadded)
                 add(data.name)
                 add(data.premiereDate?.let { formatDateTime(it) })
-            }.filterNotNull().joinToString(" - ")
+            }.joinNotBlank(" - ")
         } else {
             data.productionYear?.toString()
         }