Speed up namespace landing data loading

[shangyian]

Summary

A batch of independent optimizations to how the namespace landing page (NamespacePage) fetches and renders its data.

DJService.js

• users() drops with_activity: The landing page and owner/edited-by dropdowns only use usernames, but with_activity=true made the endpoint join the entire history table.
• listNamespacesWithGit caching: the namespace list is cached for 60s with an invalidateNamespacesCache() escape hatch, so re-entering namespace pages doesn't refetch it every mount.
• nodeTypeCounts uses the new generic nodeCounts(groupBy: TYPE) query (one grouped scan) instead of 6 aliased per-type counts.

NamespacePage/index.jsx

• Lazy load filter data: users and tags (for the Edited By / Owner / Tags dropdowns) are fetched on first dropdown open, not on mount. They still fetch eagerly when a matching filter is already active in the URL, so a pre-applied filter renders its label.
• Gate table + count fetches on gitConfigLoaded: for git-root namespaces the table used to fetch a provisional namespace, then refetch after the redirect to the default branch. Waiting for git config resolves that duplicate.
• Filters mount-guard: the URL to filters sync effect no longer refetches on first mount (state is already seeded from the URL).
• Invalidate the namespace cache when a sub-namespace is created.

CompactSelect.jsx

Add a windowed menu list (cap rendered options at 100) so large lists like the full user list don't make the dropdown super slow and unusable.

Test Plan

• PR has an associated issue: #
• make check passes
• make test shows 100% unit test coverage

Deployment Plan

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