refactor(web): reuse shared error alerts

[juliusmarminge]

Retained in the V2 UI stack. The migration includes the ACP registry screen and onboarding changes currently present only on the V2 branch.

Pairing, onboarding, provider discovery, theme search, and connection settings hand-styled error banners. They now compose the existing Alert and AlertDescription primitives. Search live-region semantics and onboarding error references are preserved.

Validation: authBootstrap, hostedPairing, AcpRegistrySearchStep, and ConnectionsSettings.logic tests, affected client typecheck, scoped lint, and React Doctor review. React Doctor’s existing consumer findings remain documented from the original audit; no new component findings were identified.

Code-only verification was requested; no browser/device pass or before/after screenshots are included. This remains a draft for visual review. No wire contracts, persisted data, or upgrade migrations change.

Model: GPT-6. Harness: Codex.

[github-actions]

Thread transfer impact

⚠️ The latest CI run did not produce a thread transfer result for dfb7e35.

This comment will update automatically after the next completed run.

[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — This is a contained UI refactor that standardizes error rendering without changing pairing, search, connection, or data-flow behavior. Human review is still required because the change includes an auth-directory file, which is treated as sensitive regardless of the narrow presentation scope.

No code changes detected at dfb7e35. Prior analysis still applies.

^{You can add or adjust custom eligibility rules. Learn more.}