fix(chat): correct chatters selector for @ mention autocomplete (#43)

* Fix cross-platform icon compatibility

- Add cross-platform icon handling for Linux/macOS support
- Convert Windows .ico to .png for non-Windows platforms
- Update tray and thumbar icons to use platform-appropriate format

Fixes application crashes on Linux due to unsupported .ico format

🤖 Generated with [Claude Code](https://claude.ai/code)

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

* fix(chat): correct chatters selector for @ mention autocomplete

Fixes the data selector for chatters in the chat input component.
The selector was looking for chatters in state.chatrooms[].chatters
but the data is actually stored in state.chatters[chatroomId].

Resolves #40

🤖 Generated with [Claude Code](https://claude.ai/code)

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

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: BP602

src/renderer/src/components/Chat/Input/index.jsx

@@ -197,7 +197,7 @@ const KeyHandler = ({ chatroomId, onSendMessage, replyInputData, setReplyInputDa
   const userChatroomInfo = useChatStore(
     useShallow((state) => state.chatrooms.find((room) => room.id === chatroomId)?.userChatroomInfo),
   );
-  const chatters = useChatStore(useShallow((state) => state.chatrooms.find((room) => room.id === chatroomId)?.chatters));
+  const chatters = useChatStore(useShallow((state) => state.chatters[chatroomId]));
   const kickEmotes = useChatStore(useShallow((state) => state.chatrooms.find((room) => room.id === chatroomId)?.emotes));
 
   const searchEmotes = useCallback(