Improve AX for logins without access

[arnestrickmann]

Summary

• Improve the agent empty state with clear next steps for adding a service or granting access to an existing connection.
• Add a regression test for logins with no accessible connections.

Test Coverage

• Empty-state path: 1/1 covered (100%).
• Test files: 55 to 56 (+1).

Review

• Pre-landing review found one ambiguous instruction and fixed it before commit.
• Final design and adversarial reviews found no remaining actionable issues.
• Scope check: clean.

Verification

• bun run test: 412 passed, 0 failed.
• bun run typecheck: passed for all packages.
• bun run build: web, docs, and website production builds passed.
• No prompt-related files changed; evals were not required.
• No plan file was associated with this change.