feat(rpc): classify RPC failures from structured data, not message text

Every caller guessed what went wrong from what(): "Timeout" meant busy,
"401" meant auth, and warmup was code -28 or one of six substrings. A
non-JSON HTTP 500 lost its body, so "Work queue depth exceeded" read as a
bare "HTTP 500", and libcurl reports connect- and reply-phase timeouts
with the same code.

New header-only src/rpc/rpc_failure.h:
- RpcFailureKind (NotListening, ConnectTimeout, ReplyTimeout, QueueFull,
  ShuttingDown, Unhandled, Auth, Warmup, AppError, Aborted, Other) and a
  pure classifyRpcFailure(curl code, request-sent, HTTP status, JSON-RPC
  code, plain-text body)
- formatHttpError keeps a short, printable plain-text body
- RpcError (moved here) stays "the daemon answered with a JSON-RPC
  error". The new RpcTransportError means "no JSON-RPC answer": curl
  failures and HTTP-layer refusals (401, 503, queue full, unhandled).
  Keeping them separate matters: callers catch RpcError to mean "the
  daemon said no" (chat identity's no-mnemonic fallback, seed export),
  so a timeout must not be one. They now treat a queue-full 500 as
  transient rather than as "no".

RPCClient:
- performCall tells the two timeouts apart with CURLINFO_REQUEST_SIZE
  (0 = never reached the daemon; reset on every transfer, checked on
  libcurl 7.81), and callRaw now shares performCall and the HTTP-failure
  path.
- connect() decides warmup by the -28 code only, so a plain-text body
  saying "Loading" can't pass for warmup, and records
  getLastConnectFailureKind().
- "Connection failed" names the kind and is logged once, then every
  100th repeat (one incident logged it 60k times).
- secret-bearing calls never echo a response body.

tryConnect's "401"/"Timeout" checks now use the kind, and
CoreRefreshResult carries blockchainErrKind/balanceErrKind for the busy
handling to come.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/services/network_refresh_service.h

@@ -4,6 +4,7 @@
 #include "data/wallet_state.h"
 #include "refresh_scheduler.h"
 #include "rpc/rpc_worker.h"
+#include "rpc/rpc_failure.h"
 
 #include <nlohmann/json.hpp>
 
@@ -112,6 +113,10 @@ public:
         std::optional<int> longestChain;
         std::optional<int> notarized;
         double balanceScanMs = 0.0;   // wall-clock spent in z_gettotalbalance this refresh (0 if balance skipped)
+        // How those calls failed (None when they succeeded or weren't made), so a caller can tell a busy
+        // node (ReplyTimeout / QueueFull) from a gone one (NotListening) without reading what().
+        rpc::RpcFailureKind blockchainErrKind = rpc::RpcFailureKind::None;
+        rpc::RpcFailureKind balanceErrKind = rpc::RpcFailureKind::None;
     };
 
     struct MiningRefreshResult {