feat(ui): add aggregated Policies page

[yupanzi]

Summary

Add a cluster-scoped aggregated Policies page:

• Aggregates all six admission-policy types (validating/mutating admission policies, their bindings, webhook configurations) into one list with a kind filter, search, and a partial-error notice when some types fail to load (e.g. the API group is not enabled on the cluster).
• The catalog entry is marked synthetic: it is a frontend aggregation, not a real API resource. The individual policy types lose their sidebar entries in favor of the aggregated page but keep their routes and detail pages.
• rbac-dialog excludes synthetic entries from resource suggestions since they cannot be RBAC targets.

Frontend-only; no backend changes.

Why

The admission-policy resources each had their own hidden-by-default sidebar entry, so getting an overview of what admission control is active in a cluster meant visiting six separate lists.

Related issue

Closes #669

Validation

• pnpm run type-check && pnpm run lint && pnpm run test
• Verified against a cluster with and without the ValidatingAdmissionPolicy API group enabled (partial-error path)

Checklist

• I reviewed this PR myself before requesting review.
• I understand the changes, including AI-generated parts (if any).
• For new features, a feature request issue is linked.
• I cleaned up AI noise (unnecessary comments, dead code, and unrelated changes).
• This PR is reasonably scoped (or split into smaller PRs).

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