More reliable sync and restore, clearer subscription info

Places that used to fail silently now speak up, and we fixed spots where what you were told could drift from what was true.

• When sync repeatedly fails and a change can't be uploaded, you now get a notification instead of silence
• Fixed billing guidance that could mismatch your actual subscription state, or suggest an unnecessary re-subscription
• Fixed open-ended events gaining an end date when moved on the calendar, and corrected time labels on events crossing midnight
• Fixed backup restore miscounting missing attachments, and partial results appearing as complete
• Fixed demo-page input possibly lingering into account data, and restore stopping because of edit-history entries
• Fixed AI date moves dropping the time and end information of an event
• Fixed device sign-out occasionally failing silently, and made the security copy match actual behavior
• Android app (in development): attachment deletion now asks before permanently removing, plus polish for short-event moves and table editing
• Plus a number of additional stability, security, and observability improvements