AI query accuracy, voice privacy, and billing math improvements

A round focused on AI honesty and data privacy.

• When you ask the AI about a category that doesn't exist, it now says so instead of returning arbitrary results — and when a list is cut short, it tells you. It can also count records by title prefix accurately
• Voice attachment transcripts are now purged from the server immediately after use
• Fixed recurring events that could appear shifted by a day under certain timezone settings
• Fixed partial-refund month math for annual billing that could drift on end-of-month billing dates
• Polished the accuracy of trash-restore, subscription-change, and device-release messages
• Plus broader server-side validation and security hardening