feat: Background iMessage sync with delivery status

[kody-w]

Parent: #22

Flow

1. User sends via twin UI → message stored encrypted → instant display
2. Background worker picks up pending messages → sends via AppleScript
3. Status updates: pending → sent → delivered (if read receipt available)
4. Incoming iMessage messages → polled via sqlite → added to encrypted store → pushed to twin UI

Edge cases

• iMessage unavailable (not macOS): twin works standalone, no sync
• iMessage send fails: retry with backoff, show error in twin UI
• Duplicate detection: message ID matching between twin store and iMessage DB

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