feat(live): refresh the channel list on demand and on resume

Live categories and channels were fetched once per app session and kept
in memory until the process died, so a TV box left running never picked
up provider changes.

- Live tab: pull-to-refresh plus a remote-friendly refresh button, and an
  automatic refetch when the app returns to the foreground after 4 hours.
- Settings: the "refresh cache" entry was a no-op delay; it now forces a
  reload of live channels, movies, series and EPG.
- The Xtream service keeps the last non-empty live lists and serves them
  when a refetch fails, so a refresh on a not-yet-connected network does
  not blank the screen.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

lib/features/iptv/services/xtream_service_mobile.dart

@@ -98,6 +98,14 @@ class XtreamServiceMobile {
   /// changing any call site.
   final Map<String, model.Channel> _channelIndex = {};
 
+  /// Last non-empty live lists, keyed by category id ('' = all channels).
+  ///
+  /// The fetchers swallow errors and return `[]`, so a refresh attempted while
+  /// the network is down (typically right after a TV wakes from standby) would
+  /// otherwise replace a good list with an empty screen.
+  final Map<String, List<model.Channel>> _lastLiveChannels = {};
+  List<model.Category>? _lastLiveCategories;
+
   late final XmltvService _xmltv = XmltvService(cacheDir);
 
   /// User-provided XMLTV URL (empty = not configured).
@@ -200,7 +208,18 @@ class XtreamServiceMobile {
 
   /// Get live channels for a category (with batch EPG support)
   /// If categoryId is empty, fetches ALL channels from all categories
+  ///
+  /// Falls back to the last good list for this category when the fetch fails.
   Future<List<model.Channel>> getLiveChannels(String categoryId) async {
+    final channels = await _fetchLiveChannels(categoryId);
+    if (channels.isNotEmpty) {
+      _lastLiveChannels[categoryId] = channels;
+      return channels;
+    }
+    return _lastLiveChannels[categoryId] ?? channels;
+  }
+
+  Future<List<model.Channel>> _fetchLiveChannels(String categoryId) async {
     try {
       final queryParams = {
         'username': _username,
@@ -463,7 +482,18 @@ class XtreamServiceMobile {
   }
 
   /// Get live TV categories
+  ///
+  /// Falls back to the last good list when the fetch fails.
   Future<List<model.Category>> getLiveCategories() async {
+    final categories = await _fetchLiveCategories();
+    if (categories.isNotEmpty) {
+      _lastLiveCategories = categories;
+      return categories;
+    }
+    return _lastLiveCategories ?? categories;
+  }
+
+  Future<List<model.Category>> _fetchLiveCategories() async {
     try {
       if (kDebugMode) print('🔍 Loading live TV categories with 8s timeout...');