* fix(sessions): throttle automatic retries after failed scans

Track Sessions attempts independently from successful snapshots so popup and tab reentry cannot bypass the automatic retry interval. Preserve manual retry, source invalidation, and last successful data. Add regression coverage for failed responses, timeout, timer boundaries, and source changes.

Documentation impact: update usage, development, and Unreleased. Existing settings, translations, package metadata, and TODO CLI blockers are unchanged.

* fix(sessions): start retry cooldown after scan completion

Record the reply or timeout finish time separately from successful snapshots. Advance the test clock through long scans and verify cooldown expiry, manual retries, and ignored late replies. Update usage and development guides; the existing Unreleased entry covers this correction and TODO scope is unchanged.