Add Slack integration and improve plugin setup flow

[arefbehboudi]

I have been thinking about improving integrations and the plugin setup experience in Javaclaw.

Slack Integration

It would be great to add integration with Slack, similar to existing platforms like Telegram or Discord.

Plugin Setup in Onboarding

Currently, onboarding includes multiple steps for setting up different plugins (Telegram, Discord, etc.), which makes the process longer and more complex.

I suggest simplifying this by:

• Having a single "Add Plugin" step during onboarding
• Providing a dropdown where users can select a plugin and configure it
• Allowing users to add multiple plugins from the same step (one after another)

This would make onboarding shorter and more user-friendly.

Plugin Management Page

In addition, I think we need a dedicated page where users can:

• View the list of added plugins
• Add new plugins
• Edit existing ones
• Remove plugins

Proposal

• Add Slack as a new integration
• Refactor onboarding to use a single, flexible plugin setup step
• Add a plugin management page in the UI

@rdehuyss I am happy to work on this if it sounds good.