Fix: Scheduled backup time changes after app restart

[patricioxavier8]

Problem

When a user configured a scheduled backup (e.g., hourly at 20:07), closing and reopening the app would cause the next backup time to be recalculated from the current time (e.g., 21:09 instead of 21:07). This happened because only the last completed backup time was persisted, not the scheduled next backup time.

Solution

• Added NEXT_SCHEDULED_BACKUP_KEY to persist the next scheduled backup date
• Modified startBackupTimer() to first check for a saved scheduled time before recalculating
• Clear the scheduled backup key when:
  • A backup completes (so next schedule is based on completion time)
  • User changes the backup frequency