fix(home): scope topology preview legend by ns + count restricted ns (#655)

[nadaverell]

Summary

Two of the three follow-ups from #655 (per-user RBAC visual-test pass on #609). Item #2 from that issue is intentionally left out — it's a product call, not a clear fix.

1. Home topology preview: legend stayed cluster-wide when a namespace was picked

The SSE topology stream is cluster-wide for small/medium clusters; only the dashboard summary (the "X resources · Y conn" header) is namespace-filtered server-side. So picking a namespace updated the header but left the kind legend reflecting cluster-wide totals.

Verified end-to-end on prod-cluster-us-east1 with the autopush namespace selected:

	Header	Legend
Pre-fix	4 resources · 2 conn	65 Deployment · 37 DaemonSet · 5 StatefulSet · 70 Service · 16 Ingress · 1 Gateway · 8 HTTPRoute · 133 Pod … (cluster-wide)
Post-fix	4 resources · 2 conn	1 Deployment · 1 Pod · 1 ConfigMap · 1 GatewayClass (matches header)

Fix: a scopedTopology memo in HomeView filters the topology to the active namespace set on the client (cluster-scoped nodes pass through, dangling edges dropped) before passing it to TopologyPreview and ActivitySummary. No-op on large clusters (≥1000 nodes) where forceNamespaceFilter already filters SSE server-side.

2. Cluster info card: "0 namespaces" for restricted users

dashboard.go only populated ResourceCounts.Namespaces when the user could list namespaces cluster-wide. For users restricted to N specific namespaces (no cluster-wide SAR), the count stayed at 0 — misleading. Fall back to len(allowedNamespaces), mirroring the same pattern already in resource_counts.go.

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Note

Low Risk
Low risk: changes are limited to dashboard display logic (client-side topology filtering) and a small server-side fallback for namespace counts, with no auth or data-write behavior modified.

Overview
Fixes mismatches on the Home dashboard when a namespace filter is active by client-filtering the SSE topology before rendering, so TopologyPreview and ActivitySummary legends/edges align with the namespace-scoped summary.

Also corrects the cluster info/resource counts for RBAC-restricted users by reporting the number of accessible namespaces when the user cannot list namespaces cluster-wide (instead of showing 0).

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