feat(network): a clear auth-failed node banner with a fix-it action

When the node rejects the wallet's RPC login (bad rpcuser/rpcpassword), the
status bar showed developer jargon ("Auth failed — check rpcuser/rpcpassword")
and the only action was Reconnect, which just repeats the failure. Now a
dedicated AuthFailed banner reason surfaces "Wallet can't sign in to the node"
with a plain-language detail and a real fix: Restart node for the embedded
daemon (regenerates its cookie creds) or Open config folder for an external one
(to fix DRAGONX.conf). Driven by a new rpc_auth_failed_ flag (set on the auth
path, cleared on each connect attempt), with evaluator unit tests.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app_network.cpp

@@ -406,6 +406,7 @@ void App::tryConnect()
     ++connect_attempt;
 
     connection_in_progress_ = true;
+    rpc_auth_failed_ = false;   // re-arm: a fresh connect attempt clears any prior auth failure
     connection_status_ = TR("sb_loading_config");
     
     // Auto-detect configuration (file I/O — fast, safe on main thread)
@@ -608,6 +609,7 @@ void App::tryConnect()
                                 } else {
                                     state_.connected = false;
                                     connection_status_ = TR("sb_auth_failed");
+                                    rpc_auth_failed_ = true;
                                     VERBOSE_LOGF("[connect #%d] .cookie auth also failed\n", attempt);
                                     ui::Notifications::instance().error(
                                         "RPC authentication failed (HTTP 401). "
@@ -621,6 +623,7 @@ void App::tryConnect()
                     state_.connected = false;
                     std::string confPath = rpc::Connection::getDefaultConfPath();
                     connection_status_ = TR("sb_auth_failed");
+                    rpc_auth_failed_ = true;
                     VERBOSE_LOGF("[connect #%d] HTTP 401 — rpcuser/rpcpassword in %s don't match the daemon. "
                                "Edit the file or restart the daemon to regenerate credentials.\n",
                                attempt, confPath.c_str());