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.h

@@ -1559,6 +1559,7 @@ private:
 
     // Connection
     std::string connection_status_ = "Disconnected";
+    bool rpc_auth_failed_ = false;   // last connect hit an auth failure (bad rpcuser/rpcpassword); drives the banner
     bool connection_in_progress_ = false;
     bool remote_rpc_plaintext_warning_shown_ = false;
     // Startup daemon-launch diagnostics: bound the "RPC port busy, no config" wait before warning,