Available MCP Servers: quick-link accounts from warning state and details view

- Motivation:
Reduce friction by allowing users to link accounts directly when an account is missing.

- Proposed solution:
In `frontend/src/app/(dashboard)/available-mcp-servers/page.tsx`, expose a clear action to open `AddAccountDialog` wherever a no-account warning appears; also provide a link from each server’s details view/modal.

- Additional context:
- Hooks: `frontend/src/features/connected-accounts/hooks/use-connected-account.tsx`.