feat: Add chat export functionality

[zhaojiyuan6]

This commit introduces a new feature allowing you to export chat records.

Key changes:

• Added an "Export Chat Records" option to the main menu.
• Implemented a multi-level submenu for selecting a chat and then choosing an export period (today or last 7 days).
• Chat messages are fetched from the database for the specified talker and date range.
• The messages are formatted and saved as a .txt file in a 'chatlog_exports' directory within your home directory.
• Filenames are generated based on the talker's display name and the export date range, with sanitization for invalid characters.
• You will receive on-screen notifications about the outcome of the export process, including the path to the saved file on success, or error messages if issues occur.