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/de.json

@@ -534,6 +534,7 @@
     "market_btc_price": "BTC PREIS",
     "market_cap": "Marktkapitalisierung",
     "market_cap_short": "Kap.",
+    "market_chart_loading": "Preisverlauf wird geladen",
     "market_iv_1d": "1T",
     "market_iv_1h": "1S",
     "market_iv_1m": "1M",

res/lang/es.json

@@ -534,6 +534,7 @@
     "market_btc_price": "PRECIO BTC",
     "market_cap": "Cap. de Mercado",
     "market_cap_short": "Cap.",
+    "market_chart_loading": "Cargando historial de precios",
     "market_iv_1d": "1D",
     "market_iv_1h": "1H",
     "market_iv_1m": "1M",

res/lang/fr.json

@@ -534,6 +534,7 @@
     "market_btc_price": "PRIX BTC",
     "market_cap": "Capitalisation",
     "market_cap_short": "Cap.",
+    "market_chart_loading": "Chargement de l'historique des prix",
     "market_iv_1d": "1J",
     "market_iv_1h": "1H",
     "market_iv_1m": "1M",

res/lang/ja.json

@@ -534,6 +534,7 @@
     "market_btc_price": "BTC価格",
     "market_cap": "時価総額",
     "market_cap_short": "時価総額",
+    "market_chart_loading": "価格履歴を読み込み中",
     "market_iv_1d": "1日",
     "market_iv_1h": "1時間",
     "market_iv_1m": "1ヶ月",

res/lang/ko.json

@@ -534,6 +534,7 @@
     "market_btc_price": "BTC 가격",
     "market_cap": "시가총액",
     "market_cap_short": "시총",
+    "market_chart_loading": "가격 기록 불러오는 중",
     "market_iv_1d": "1일",
     "market_iv_1h": "1시간",
     "market_iv_1m": "1개월",

res/lang/pt.json

@@ -534,6 +534,7 @@
     "market_btc_price": "PREÇO BTC",
     "market_cap": "Capitalização",
     "market_cap_short": "Cap.",
+    "market_chart_loading": "Carregando histórico de preços",
     "market_iv_1d": "1D",
     "market_iv_1h": "1H",
     "market_iv_1m": "1M",

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",

res/lang/zh.json

@@ -534,6 +534,7 @@
     "market_btc_price": "BTC 价格",
     "market_cap": "市值",
     "market_cap_short": "市值",
+    "market_chart_loading": "正在加载价格历史",
     "market_iv_1d": "1天",
     "market_iv_1h": "1时",
     "market_iv_1m": "1M",