AI response language and document upload fixes, voice privacy improvement

A pass over the AI's language behavior, document learning, and voice data handling.

• Fixed the AI assistant replying in a language different from your app language setting — it now follows your setting
• Improved speech recognition so it identifies the spoken language more accurately
• Fixed a problem where adding documents to the AI's knowledge could fail
• Moving to a new device now shows a confirmation first, with a heads-up that it uses one of your monthly changes
• Server-side transcripts of voice recordings are now deleted automatically after 7 days — notes saved on your device are unaffected
• Plus general stability improvements