Feature: Implement multi-account support

[nurefexc]

Goal

Enable the extension to monitor multiple Matrix accounts simultaneously. This is essential for users who maintain separate accounts for professional and personal use across different homeservers.

Technical Context

The current architecture assumes a single homeserver_url and access_token. This needs to be refactored to handle an array of credentials and aggregate unread counts/room lists from multiple sources.

Tasks

• Refactor the settings schema to store multiple account configurations.
• Update the /sync loop to iterate through all configured accounts asynchronously.
• Implement account-based filtering or grouping in the room list UI.
• Ensure the "Preferred Client" logic correctly handles account-specific room IDs.

Trade-offs

Increased network traffic and resource usage. We need to ensure that multiple sync loops don't degrade GNOME Shell performance.