v1.8.7 — fix update-toast cascade + mute during playback

Two community-reported issues fixed: (1) Installing a new plugin version replayed every unlocked achievement as a toast — fixed by stamping startup backfill with Unix-epoch so those unlocks are always older than any client's last-seen cutoff. Also persists the per-browser shown-IDs in localStorage instead of sessionStorage so a refresh doesn't replay them. (2) Two new user prefs (default ON): mute achievement toasts during playback + mute unlock sound during playback. Both can be toggled independently in the standalone settings panel.