feat(tvos): fetch Top Shelf content live and show poster art

The Top Shelf extension now fetches Continue Watching directly from
Plex/Jellyfin/Emby instead of replaying a cache the app wrote on its
last foreground Discover pass. The app publishes per-profile server
descriptors on every shelf sync (`updateSources`): token-free metadata
in the app group, tokens in an app-group-shared keychain item, both
wiped by `clear`. On success the extension rewrites the cached payload
as the offline fallback; any fetch failure falls back to the previous
cache-replay behavior. Poster images are passed as remote URLs, so the
extension no longer depends on app-side artwork downloads.

Episodes now render season/series poster art (2:3, `.poster` shape)
instead of 16:9 episode stills, and labels lead with the S/E marker so
long titles no longer hide it behind the focused-item marquee. Shelf
schema v3 (Dart, Android, tvOS envelopes bumped together) discards
stale wide-art caches instead of letterboxing them into poster slots.

close #1474
close #1835

android/app/src/main/kotlin/com/edde746/plezy/watchnext/WatchNextPlugin.kt

@@ -24,7 +24,7 @@ class WatchNextPlugin() :
   companion object {
     private const val TAG = "WatchNextPlugin"
     private const val METHOD_CHANNEL = "com.plezy/watch_next"
-    private const val SCHEMA_VERSION = 2
+    internal const val SCHEMA_VERSION = 3
     private var pendingDeepLink: String? = null
 
     fun handleIntent(intent: Intent?): String? {

android/app/src/test/kotlin/com/edde746/plezy/watchnext/WatchNextProviderTest.kt

@@ -291,7 +291,7 @@ class WatchNextProviderTest {
       MethodCall(
         "sync",
         mapOf(
-          "schemaVersion" to 2,
+          "schemaVersion" to 3,
           "ownerId" to "active-owner",
           "generation" to 1L,
           "items" to listOf(mapOf("contentId" to "active", "title" to "Active"))
@@ -308,7 +308,7 @@ class WatchNextProviderTest {
         MethodCall(
           "sync",
           mapOf(
-            "schemaVersion" to 2,
+            "schemaVersion" to 3,
             "ownerId" to "queued-owner",
             "generation" to 2L,
             "items" to listOf(mapOf("contentId" to "queued", "title" to "Queued"))
@@ -1281,7 +1281,7 @@ class WatchNextProviderTest {
   private fun syncCall(ownerId: String, generation: Long) = MethodCall(
     "sync",
     mapOf(
-      "schemaVersion" to 2,
+      "schemaVersion" to 3,
       "ownerId" to ownerId,
       "generation" to generation,
       "items" to emptyList<Map<String, Any?>>()