Implements a service-worker-side session timer that locks the vault after a configurable period of inactivity (default 15 min). Supports two modes: 'inactivity' (timer-based) and 'every_time' (no timer). Config persists via chrome.storage.local and is exposed through get_session_config / update_session_config popup messages. Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>