feat(chat): add bubbles for shared chat participant channels (#581)

* feat(chat): add bubbles for shared chat participant channels

Twitch's native shared chat previously had no way to interact with
participant channels the way manually-added chats work. Adds
per-participant bubbles alongside manual tabs: tapping one spotlights
that channel (fading the rest), hiding one filters its messages from
the feed entirely. Defaults to unfaded, resets on rejoin (fresh
ChatStore per connect).

Manual "merged mode" fading is untouched and unaffected.

* 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

* fix(chat): don't spotlight a hidden shared-chat bubble

toggleFocus didn't check hiddenChannelIds, so tapping a hidden
bubble's chip added it to the spotlight set — fading every other
participant to showcase a channel whose own messages were already
filtered out of the feed.

* fix(chat): stop shared-chat detection flapping on system messages

_isInSharedChatMode was recomputed from every PRIVMSG/NOTICE/USERNOTICE,
but only PRIVMSG reliably carries source-room-id for a session's
duration. A sub/raid/announcement without the tag would flip the flag
off and back on within a single live session. Previously this only
risked a redundant asset refetch (already guarded); now the shared
chat bubble bar reads the flag directly, so a flap would visibly
collapse and reopen it. Scope detection to PRIVMSG only.

test/screens/channel/chat/shared_chat_bubble_state_test.dart

@@ -0,0 +1,57 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:frosty/screens/channel/chat/stores/shared_chat_bubble_state.dart';
+
+void main() {
+  late SharedChatBubbleState state;
+  setUp(() => state = SharedChatBubbleState());
+
+  test('a channel is neither focused nor hidden by default', () {
+    expect(state.isFocused('1'), isFalse);
+    expect(state.isHidden('1'), isFalse);
+  });
+
+  test('toggleFocus spotlights an unfocused channel', () {
+    state.toggleFocus('1');
+    expect(state.isFocused('1'), isTrue);
+  });
+
+  test('toggleFocus un-spotlights an already-focused channel', () {
+    state.toggleFocus('1');
+    state.toggleFocus('1');
+    expect(state.isFocused('1'), isFalse);
+  });
+
+  test('multiple channels can be spotlighted at once', () {
+    state.toggleFocus('1');
+    state.toggleFocus('2');
+    expect(state.isFocused('1'), isTrue);
+    expect(state.isFocused('2'), isTrue);
+  });
+
+  test('hide filters a channel and drops it from the spotlight', () {
+    state.toggleFocus('1');
+    state.hide('1');
+    expect(state.isHidden('1'), isTrue);
+    expect(state.isFocused('1'), isFalse);
+  });
+
+  test('toggleFocus is a no-op for a hidden channel', () {
+    state.hide('1');
+    state.toggleFocus('1');
+    expect(state.isFocused('1'), isFalse);
+  });
+
+  test('show lifts a hide', () {
+    state.hide('1');
+    state.show('1');
+    expect(state.isHidden('1'), isFalse);
+  });
+
+  test('reset clears both focused and hidden channels', () {
+    state.toggleFocus('1');
+    state.hide('2');
+    state.reset();
+    expect(state.isFocused('1'), isFalse);
+    expect(state.isHidden('2'), isFalse);
+  });
+}