[FEATURE] Local conversation search and JSON export (complementing clear history)

[krwg]

Summary

Incremental search across localStorage backed transcript model with debounced substring match; optional per-peer JSON export sanitized for sharing.

Background

Parity with archival expectations absent cloud sync.

Scope

Virtualized scrolling future optional — initial simple filter pass acceptable up to capped history lengths already enforced (MAX_PER_PEER).
Export excludes binary attachments (none presently) marker field schema: blip_chat_export_v1.

Acceptance criteria

• Search box filters visible transcript non-destructively.
• Export writes valid JSON reproducible round-trip importer stub optional backlog.
• Performance acceptable ≤10k msgs aggregate synthetic bench local only.

Technical notes

Maintain existing storage key versioning blip_chat_v1; future migrations isolated.

Definition of done

Clear + export interaction doesn't orphan partial files / temp blobs.