Track how long users spend studying by timing foreground sessions.
StudyTimerService starts on app active, stops on background, and
accumulates seconds into DailyLog.studySeconds (CloudKit-synced).
Dashboard shows today/total study time with a 7-day bar chart.
Closes#1
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>