Message ordering issue: No sequence enforcement

[rafabd1]

The current message handling uses the double-ratchet's internal counter (wire.n) for decryption, but doesn't enforce message ordering:

1. Counter used for decryption only: The wire.n counter is passed to ratchet.decrypt() but:

   • No check that messages arrive in increasing counter order
   • Out-of-order delivery (possible on lossy networks/I2P) would silently decrypt out-of-order

2. Timestamp only for display: Messages use unix timestamp for display, NOT for ordering:

   • Two messages sent at the same second could reorder in the UI
   • Timestamp is added CLIENT-SIDE on receive, not sent by the peer
   • If clock skew exists between peers, ordering appears wrong to each side

3. Frontend sorting: Messages in ChatWindow.tsx just render state.messages in array order. If the backend pushes out-of-order messages, they appear in wrong order.

Examples of failure:

• Peer A sends msg1 (counter=1), msg2 (counter=2)
• Network reorders: Peer B receives msg2, then msg1
• Backend pushes them in order received → UI shows msg2 before msg1
• Both peers think the other side is confused

Proposed solution:

• Add explicit sequence numbers to the message protocol
• Enforce or detect out-of-order delivery
• Buffer and reorder messages, or signal warning to user
• Consider adding sender-provided timestamps (encrypted in message) for consistency check