Hide imported conversations by default with a Show imported toggle

[witbrock]

Behaviour

Large coding-conversation imports crowd ordinary conversations out of the tray. Imported conversations are now hidden by default, with a Show imported toggle that includes them in the list and its content search for the current page session.

The existing import provenance marker is filtered before pagination. Direct opening, read-only transcripts, source custody, dates, active drafts and typed search text are preserved. No archive or data migration is performed.

Validation

• 43 targeted backend tests passed across catalogue, history, search, management and resilience.
• 9 frontend tests passed, including toggle and search propagation.
• Browser replay using the real catalogue module: default hidden, enable, disable, draft preserved, no JavaScript errors.
• Diff and targeted Python syntax checks passed.

Native task: #V#task_hide_imported_conversations_by_default_342ccb60.

Merge decision: ready once required CI passes. Narrow activation and canonical DGX read-back follow publication; the separate federation cutover remains paused.