FR-2: New Message Detection

• Compare the fetched message list against the currently cached list
• Use message GUID as the canonical identifier (not text, not timestamp)
• Detect both:
  • Append: new messages at the end (most common)
  • Insert: messages appearing in the middle (rare, but possible with iMessage sync delays)
• Do not re-fetch the entire message history on every poll — fetch only recent messages (e.g., last 10 or messages since the newest cached timestamp)