Fix Python 3.13 compatibility and add group DM support

[nullc0rp]

• Update urwid to >=3.0.3 for Python 3.13 compatibility
• Fix asyncio compatibility (replace @coroutine with async def)
• Fix channel loading: use conversations.list for channels, users.conversations for DMs
• Add pagination support for loading all messages (not just recent)
• Add support for group DMs (mpim) - fetch and display multi-person conversations
• Fix RTM connection errors appearing on screen (suppress stderr)
• Fix img2txt errors appearing on screen (redirect stderr)
• Add comprehensive logging to file (~/.sclack_logs/sclack.log)
• Fix icon rendering: replace private-use Unicode with standard characters
• Fix recursion issue in ChatBox.header setter
• Improve error handling throughout (check API response 'ok' status)
• Update setup.py dependencies to match requirements.txt
• Add INSTALL.md with installation instructions for pipx/Arch Linux