Add branch landing page and switcher for repo-backed namespaces

[shangyian]

Summary

When viewing a git-root namespace (one that backs a repo but is not itself a branch), the namespace page now shows a branch landing view instead of the flat node table: branch cards with node counts, invalid counts, and last-deployed timestamps, plus a preview of the default branch's nodes grouped by type (metrics, cubes, dimensions, transforms).

When viewing a branch namespace, the last breadcrumb segment becomes a dropdown that lets you switch directly to any other branch. Node names in branch namespace tables are shown relative to the namespace (e.g. revenue instead of acme.main.revenue).

Test Plan

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

Deployment Plan

[netlify]

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