Completion cues. This directory is asset-globbed in pubspec.yaml, so adding a
file here is all it takes to bundle it.

bubble_button.wav   The check-off pop. Plays for every completed task and for
                    the habit check-in that finishes the day.
goal_complete.wav   The goal celebration chime, played by the wax-seal overlay.

streak_milestone.wav (not present)
                    Optional slot for habit streak milestones. See
                    AudioService.playStreakMilestone() for what plays without
                    it; dropping a short, warm cue here needs no code change.

All cues play at 30% volume through a silent-mode-aware AudioContext, and the
"Completion sounds" setting mutes them all.
