feat(market): flag a stale price instead of showing it as a fresh green number

The aggregate price is deliberately kept on a failed or old refresh (it's never
cleared), so the hero number could be hours out of date while still drawn in the
"live" green. It now turns amber with an "as of N min/h ago" caption and a hover
tooltip carrying the friendly fetch error, when the last refresh failed or the
aggregate is >=5 min old (only when the hero is the aggregate, not an exchange
pair with its own feed). The hardcoded "Price fetch failed" default is now i18n.

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

src/services/network_refresh_service.cpp

@@ -2,6 +2,7 @@
 #include "history_scan_policy.h"
 #include "rpc/rpc_busy_gate.h"
 #include "../util/logger.h"
+#include "../util/i18n.h"   // util::tr for the price-fetch-failed default
 
 #include <algorithm>
 #include <cctype>
@@ -1995,7 +1996,7 @@ void NetworkRefreshService::applyPriceRefreshFailure(WalletState& state,
 {
     state.market.price_loading = false;
     state.market.price_error = errorMessage.empty()
-        ? std::string("Price fetch failed")
+        ? std::string(util::tr("market_price_fetch_failed"))
         : errorMessage;
 }