refactor(chat): dedup shared-chat tag lookup and extract fade policy

- add IRCMessage.sourceChannelId, replacing three copies of the same
  source-room-id/room-id tag lookup
- move the shared-chat spotlight/merged-mode fade decision out of
  ChatMessage's build into ChatStore.shouldFadeMessage, matching the
  extract-into-store convention already used for SharedChatBubbleState
- factor the tab-bar item's Padding/Center wrapper into a shared
  _wrapItem helper instead of duplicating it per branch

lib/models/irc.dart

@@ -56,6 +56,11 @@ class IRCMessage {
     this.actionLabel,
   });
 
+  /// The channel this message actually originated from. In Twitch shared
+  /// chat, `source-room-id` names the participant channel a message was
+  /// sent in; otherwise it's just this connection's own room.
+  String? get sourceChannelId => tags['source-room-id'] ?? tags['room-id'];
+
   /// Applies the given CLEARCHAT message to the provided messages and buffer.
   static void clearChat({
     required List<IRCMessage> messages,
@@ -186,7 +191,6 @@ class IRCMessage {
       span.add(const TextSpan(text: ' '));
     }
 
-    final sourceChannelId = tags['source-room-id'] ?? tags['room-id'];
     final sourceChannelUser = channelIdToUserTwitch?[sourceChannelId];
     if (sourceChannelUser != null) {
       final isCurrentChannel = sourceChannelId == currentChannelId;

lib/screens/channel/chat/stores/chat_store.dart

@@ -211,6 +211,31 @@ abstract class ChatStoreBase with Store {
     revealedMessageIds.add(id);
   }
 
+  /// Whether a message from [sourceChannelId] should render faded.
+  ///
+  /// [overrideCurrentChannelId] is set only when rendering the manual
+  /// "merged mode" view (multiple manually-added tabs interleaved), which
+  /// always fades non-active-tab messages, gated by [focusCurrentChannel].
+  /// Otherwise this is the normal single-connection view, where native
+  /// Twitch shared chat defaults to unfaded and only fades once the user
+  /// spotlights a participant channel via a chat bubble — an explicit
+  /// action that isn't gated by that setting.
+  bool shouldFadeMessage({
+    required String? sourceChannelId,
+    required String? overrideCurrentChannelId,
+  }) {
+    if (sourceChannelId == null) return false;
+
+    final currentChannelId = overrideCurrentChannelId ?? channelId;
+    if (sourceChannelId == currentChannelId) return false;
+
+    final isMerged = overrideCurrentChannelId != null;
+    if (isMerged) return settings.focusCurrentChannel;
+
+    return sharedChatBubbles.focusedChannelIds.isNotEmpty &&
+        !sharedChatBubbles.isFocused(sourceChannelId);
+  }
+
   /// Timer used for dismissing the notification.
   Timer? _notificationTimer;
 
@@ -255,8 +280,7 @@ abstract class ChatStoreBase with Store {
     // has hidden. Short-circuit when nothing is hidden (the common case).
     if (sharedChatBubbles.hiddenChannelIds.isEmpty) return base;
     return base.where((message) {
-      final sourceChannelId =
-          message.tags['source-room-id'] ?? message.tags['room-id'];
+      final sourceChannelId = message.sourceChannelId;
       return sourceChannelId == null ||
           !sharedChatBubbles.hiddenChannelIds.contains(sourceChannelId);
     }).toList();

lib/screens/channel/chat/widgets/chat_message.dart

@@ -626,27 +626,10 @@ class ChatMessage extends StatelessWidget {
                 child: dividedMessage,
               );
 
-        final sourceChannelId =
-            ircMessage.tags['source-room-id'] ?? ircMessage.tags['room-id'];
-        final currentChannelId =
-            overrideCurrentChannelId ?? chatStore.channelId;
-
-        // overrideCurrentChannelId is only set in manual "merged mode"
-        // (multiple manually-added tabs interleaved); that keeps its
-        // existing always-fade-non-active-tab behavior, gated by the
-        // focusCurrentChannel setting. Native Twitch shared chat instead
-        // defaults to unfaded and only fades once the user spotlights a
-        // participant channel via a chat bubble — an explicit action that
-        // isn't gated by that setting.
-        final isMerged = overrideCurrentChannelId != null;
-        final hasSpotlight =
-            !isMerged && chatStore.sharedChatBubbles.focusedChannelIds.isNotEmpty;
-        final shouldFade =
-            sourceChannelId != null &&
-            sourceChannelId != currentChannelId &&
-            ((isMerged && chatStore.settings.focusCurrentChannel) ||
-                (hasSpotlight &&
-                    !chatStore.sharedChatBubbles.isFocused(sourceChannelId)));
+        final shouldFade = chatStore.shouldFadeMessage(
+          sourceChannelId: ircMessage.sourceChannelId,
+          overrideCurrentChannelId: overrideCurrentChannelId,
+        );
 
         final fadedMessage = shouldFade
             ? Opacity(opacity: 0.55, child: coloredMessage)

lib/screens/channel/chat/widgets/chat_tabs.dart

@@ -49,6 +49,16 @@ class ChatTabs extends StatelessWidget {
     }
   }
 
+  /// Wraps a tab bar item (a tab chip or a shared-chat bubble) with its
+  /// trailing gap and centering, keyed for the reorderable list.
+  Widget _wrapItem({required Key key, required bool isLast, required Widget child}) {
+    return Padding(
+      key: key,
+      padding: EdgeInsets.only(right: isLast ? 0 : 4),
+      child: Center(child: child),
+    );
+  }
+
   @override
   Widget build(BuildContext context) {
     return Observer(
@@ -163,24 +173,22 @@ class ChatTabs extends StatelessWidget {
                                       1;
                               if (index < tabs.length) {
                                 final tabInfo = chatTabsStore.tabs[index];
-                                return Padding(
+                                return _wrapItem(
                                   key: ValueKey(tabInfo.channelId),
-                                  padding: EdgeInsets.only(right: isLast ? 0 : 4),
-                                  child: Center(child: _buildTab(context, index)),
+                                  isLast: isLast,
+                                  child: _buildTab(context, index),
                                 );
                               }
 
                               final participant =
                                   sharedChatParticipants[index - tabs.length];
-                              return Padding(
+                              return _wrapItem(
                                 key: ValueKey('shared_${participant.key}'),
-                                padding: EdgeInsets.only(right: isLast ? 0 : 4),
-                                child: Center(
-                                  child: _buildSharedChatBubble(
-                                    context,
-                                    participant.key,
-                                    participant.value,
-                                  ),
+                                isLast: isLast,
+                                child: _buildSharedChatBubble(
+                                  context,
+                                  participant.key,
+                                  participant.value,
                                 ),
                               );
                             },