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-sdk/src/client.rs

@@ -92,6 +92,16 @@ impl QpqClient {
         self.session_token.is_some()
     }
 
+    /// Get the server address as a string (e.g. "127.0.0.1:7000").
+    pub fn server_addr_string(&self) -> String {
+        self.config.server_addr.to_string()
+    }
+
+    /// Get the state file path from the client configuration.
+    pub fn config_state_path(&self) -> std::path::PathBuf {
+        self.config.state_path.clone()
+    }
+
     /// Get a reference to the RPC client (for direct calls).
     pub fn rpc(&self) -> Result<&quicproquo_rpc::client::RpcClient, SdkError> {
         self.rpc.as_ref().ok_or(SdkError::NotConnected)