* fix(web): preserve composer attachments across session switches
Persist composer files per session and restore completed uploads without
uploading them again. Clear attachment drafts alongside text after send and
cover restoration, isolation, and adapter reuse with regression tests.
Fixes#465
* fix(web): keep cleared attachment drafts tombstoned
Retain an empty in-memory cache entry until the queued IndexedDB delete completes so a fast remount cannot restore stale files. Add regression coverage for the clear/remount race.
* fix(web): defer attachment restore for inactive sessions
Only restore or clear attachment drafts while the session attachment adapter is available. Preserve saved files when an inactive session mounts without attachment support and add regression coverage.
