feat(market): loading spinner while the price chart fetches (pair switches)

Switching exchange pairs (or first load) briefly leaves the chart with no series
while the venue's candles fetch, which showed the bare "No price history
available" empty state. When a chart fetch is in flight — App::isMarketChartLoading()
(the CoinGecko aggregate OR the per-exchange fetch) — draw a spinner + animated
"Loading price history…" in the plot area instead; the "no history" text only
shows when genuinely empty and idle. New i18n key market_chart_loading across all
8 languages. Verified via a forced-state render (spinner + label centered).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

res/lang/ru.json

@@ -534,6 +534,7 @@
     "market_btc_price": "ЦЕНА BTC",
     "market_cap": "Рыночная капитализация",
     "market_cap_short": "Кап.",
+    "market_chart_loading": "Загрузка истории цен",
     "market_iv_1d": "1Д",
     "market_iv_1h": "1Ч",
     "market_iv_1m": "1М",