sequenceDiagram
    actor User
    participant App as Flutter App
    participant SP as SharedPreferences

    User->>App: Opens app
    App->>SP: Check for saved username
    SP-->>App: null
    App->>User: Show UsernameScreen
    User->>App: Enters username → taps "Get Started"
    App->>SP: Save username
    App->>User: Navigate to ChatScreen

sequenceDiagram
    participant A as Alice
    participant S as Go Server
    participant B as Bob

    A->>S: WS connect /ws?username=alice
    S->>S: hub.Register("alice")
    B->>S: WS connect /ws?username=bob
    S->>S: hub.Register("bob")

    A->>S: {"type": "session:connect", "target": "bob"}
    S->>S: Create session + auto-join both

    S-->>A: {"type": "session:joined", "peer": "bob"}
    S-->>B: {"type": "session:joined", "peer": "alice"}

sequenceDiagram
    participant A as Alice
    participant S as Go Server (relay only)
    participant B as Bob

    Note over A: Generate X25519 key pair
    Note over B: Generate X25519 key pair

    A->>S: {"type": "key:exchange", pubKey_A}
    S->>B: Relay pubKey_A → Bob
    B->>S: {"type": "key:exchange", pubKey_B}
    S->>A: Relay pubKey_B → Alice

    Note over A: SharedSecret = X25519(privKey_A, pubKey_B)
    Note over B: SharedSecret = X25519(privKey_B, pubKey_A)
    Note over A,B: Both derive the same shared secret — server never sees it

sequenceDiagram
    participant A as Alice (Initiator)
    participant S as Go Server (relay)
    participant B as Bob

    A->>A: Create PeerConnection + DataChannel
    A->>A: Create SDP Offer
    A->>S: signal:offer → Bob
    S->>B: Relay offer

    B->>B: Set remote desc, create Answer
    B->>S: signal:answer → Alice
    S->>A: Relay answer

    A->>S: signal:ice → Bob
    S->>B: Relay ICE candidate
    B->>S: signal:ice → Alice
    S->>A: Relay ICE candidate

    Note over A,B: 🔒 DataChannel OPEN — P2P connected!
    Note over S: Server's job is done

sequenceDiagram
    participant A as Alice
    participant B as Bob

    A->>A: Encrypt("hello") with NaCl Box
    A->>B: DataChannel → base64(nonce + ciphertext + MAC)
    B->>B: Decrypt → "hello"
    B-->>A: Display message ✓

    Note over A,B: Messages flow directly P2P — zero server involvement