feat: Digital Twin real-time UI with instant messaging

[kody-w]

Parent: #22

Upgrade twin.html to a full real-time messaging UI that's the primary conversation view.

Features

• WebSocket connection to gateway for instant updates
• Messages appear immediately (before iMessage sync)
• Persona avatars and emoji prefixes
• Delivery status indicators: ⚡instant → 📤syncing → ✅delivered
• Conversation switcher (multiple encrypted channels)
• Copy-to-iMessage button for manual relay mode
• Key exchange UI (scan QR or paste egg.json)
• Works offline — pure encrypted messaging without iMessage

Tech

• Vanilla HTML/CSS/JS (no build step, served from /twin.html)
• WebSocket for real-time, falls back to polling
• Web Crypto API for client-side encryption/decryption

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