fix(library): surface tenant-authored reporting extensions in the taxonomy picker

[jfrench9]

Problem

Tenant-authored reporting-extension taxonomies (e.g. Driftline's disclosure members and policy text blocks, authored via the TaxonomyBlock envelope) were invisible in the Taxonomy Library — the picker only admitted reporting_standard and chart_of_accounts types. Their elements instead leaked onto the Chart of Accounts page (fixed server-side in robosystems#909), implying they were CoA accounts.

Fix

Admit reporting_extension taxonomies to the library picker when isShared === false (tenant-authored). Library-seeded extension packages (rs-gaap-disclosures, rs-metric, rs-gaap-reporting-styles) remain hidden — they're framework plumbing, not tenant content. Extension taxonomies label by name (e.g. "Driftline Reporting Extension (4)") since they carry no standard.

Also updates the page subtitle to name reporting extensions as tenant-managed content.

Notes

• Data was already available: the backend returns tenant taxonomies on tenant graphs and the client query already selects taxonomyType + isShared — this is purely a picker-filter change
• Hierarchy view degrades gracefully for extensions (no standard → existing "no arc taxonomy" message)

Companion backend PR: RoboFinSystems/robosystems#909