Align Web design notes with Provider declarations

[SaladDay]

Fixes from the batched review of #539 and #540.

• apps/web/DESIGN.md: Add node no longer repeats the docker-group warning. It is stated once, in Use Docker instead of microsandbox?. Clean up the host no longer carries the microsandbox note, because the uninstaller prints what it keeps.
• docs/sandbox-provider.md (en and zh): the known design gap lists every place Web still names providers, including E2B's configuration fields in the deployment summary and in the client's deployment projection.
• Readiness advice for provider_unavailable and host_unsupported (en and zh): adds that a manually registered node logs the local error, because those nodes have no install command to rerun.

Checks: Web typecheck; sandbox-diagnostic and locale vitest; the translation test; make check-docs.

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