feat(client): v2 REPL over SDK with categorized help and tab-completion

925-line REPL replacing the 3317-line monolith — delegates all crypto,
MLS, and RPC to quicproquo-sdk. 20 commands across 6 categories
(messaging, groups, account, keys, utility, debug), rustyline tab
completion, background event listener, auto-server-launch.

Also adds SDK accessor methods (server_addr_string, config_state_path),
WS bridge register handler, and README table formatting cleanup.

crates/quicproquo-server/src/ws_bridge.rs

@@ -165,6 +165,7 @@ async fn dispatch(state: &WsBridgeState, req: RpcRequest) -> RpcResponse {
         "send" => handle_send(state, req.id, &req.params),
         "receive" => handle_receive(state, req.id, &req.params),
         "deleteAccount" => handle_delete_account(state, req.id, &req.params),
+        "register" => handle_register(state, req.id, &req.params),
         _ => RpcResponse::error(req.id, format!("unknown method: {}", req.method)),
     }
 }
@@ -175,6 +176,89 @@ fn handle_health(id: serde_json::Value) -> RpcResponse {
     RpcResponse::success(id, serde_json::json!("ok"))
 }
 
+fn handle_register(
+    state: &WsBridgeState,
+    id: serde_json::Value,
+    params: &serde_json::Value,
+) -> RpcResponse {
+    // Only allow in insecure-auth mode (development/demo).
+    if !state.allow_insecure_auth {
+        return RpcResponse::error(id, "register is only available in --allow-insecure-auth mode");
+    }
+
+    // Rate limit.
+    let auth_ctx = match extract_auth(state, params) {
+        Ok(ctx) => ctx,
+        Err(e) => return RpcResponse::error(id, e),
+    };
+    if let Err(e) = ws_check_rate_limit(state, &auth_ctx) {
+        return RpcResponse::error(id, e);
+    }
+
+    // Validate username.
+    let username = match params.get("username").and_then(|v| v.as_str()) {
+        Some(u) if !u.is_empty() => u,
+        _ => return RpcResponse::error(id, "missing or empty 'username' param"),
+    };
+    if username.len() > 32 {
+        return RpcResponse::error(id, "username must be at most 32 characters");
+    }
+    if !username.chars().all(|c| c.is_ascii_alphanumeric() || c == '_') {
+        return RpcResponse::error(id, "username must be alphanumeric or underscore only");
+    }
+
+    // Validate identity key.
+    let ik_b64 = match params.get("identityKey").and_then(|v| v.as_str()) {
+        Some(s) if !s.is_empty() => s,
+        _ => return RpcResponse::error(id, "missing or empty 'identityKey' param"),
+    };
+    let identity_key = match B64.decode(ik_b64) {
+        Ok(k) => k,
+        Err(e) => return RpcResponse::error(id, format!("bad base64 identityKey: {e}")),
+    };
+    if identity_key.len() != 32 {
+        return RpcResponse::error(id, "identityKey must be 32 bytes");
+    }
+
+    // Check if username is already taken by a different key.
+    match state.store.get_user_identity_key(username) {
+        Ok(Some(existing)) if existing == identity_key => {
+            // Idempotent: same key, return success.
+            return RpcResponse::success(
+                id,
+                serde_json::json!({
+                    "username": username,
+                    "identityKey": B64.encode(&identity_key),
+                }),
+            );
+        }
+        Ok(Some(_)) => {
+            return RpcResponse::error(id, "username already taken");
+        }
+        Ok(None) => {} // Available, proceed.
+        Err(e) => return RpcResponse::error(id, format!("storage error: {e}")),
+    }
+
+    // Store the mapping.
+    if let Err(e) = state.store.store_user_identity_key(username, identity_key.clone()) {
+        return RpcResponse::error(id, format!("storage error: {e}"));
+    }
+
+    tracing::info!(
+        username = %username,
+        key_prefix = %hex::encode(&identity_key[..4]),
+        "audit: ws_bridge register"
+    );
+
+    RpcResponse::success(
+        id,
+        serde_json::json!({
+            "username": username,
+            "identityKey": B64.encode(&identity_key),
+        }),
+    )
+}
+
 async fn handle_resolve_user(
     state: &WsBridgeState,
     id: serde_json::Value,