No idempotency: Duplicate messages on network redelivery

[rafabd1]

The protocol has no duplicate detection. If a message is retransmitted on the I2P network (e.g., due to timeout+retry), it will be stored twice:

Wire format (messaging.rs:19-24):

struct WireMessage {
    t: &str,      // type ("msg")
    id: &str,     // UUID (unique)
    ct: String,   // ciphertext
    n: u32,       // ratchet counter
}

Problem: While id is unique per message sent, there's no idempotency check on receive:

let entry = MessageEntry {
    id: wire.id.clone(),
    // ...
};
state.messages.lock().await.push(entry);  // No check if id already exists!

Failure scenario:

1. Peer sends message with id="abc123", counter=42
2. Network delivers it, peer doesn't get ACK immediately
3. Peer retransmits with same id, counter=42
4. We receive both → push both to state.messages
5. User sees the same message twice in the UI

Why this matters:

• I2P doesn't guarantee delivery, apps must handle retransmissions
• User sees duplicate messages
• Can break message count expectations
• TTL expiration tracking gets confused

Proposed solution:

• Keep a set of recently seen message IDs (with TTL)
• Deduplicate on receive: if id already in messages, skip or update
• Or: explicit ACK mechanism (peer sends "msg_ack" with message id)