[Improve] Explain Slack access before connection

[roomote-roomote]

​Opened on behalf of @mrubens. View the task or mention @roomote-roomote for follow-up asks.

What changed

• Explain Slack access boundaries before the user starts OAuth: Roomote can work in direct messages and channels where the app is added, list public channels for destination selection, and cannot access private channels unless invited.
• Add focused component coverage for each access claim while preserving the existing connect action test.

Why this change was made

The Slack onboarding step previously said only that Roomote could “talk with your workspace,” leaving users uncertain about what channel content the app could read or post to.

Impact

Users can review the practical Slack permission boundaries before approving the connection. The focused client tests and web static checks pass; browser proof was not captured because the local onboarding flow could not pass inference configuration without an available local provider.

[roomote-community]

No code issues found. See task

_{Reviewed ac017b9}