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>

scripts/add_missing_translations.py

@@ -2254,6 +2254,12 @@ TRANSLATIONS = {
         "ja": "フォルダーをスキャンして他のウォレットを探す",
         "ko": "폴더를 스캔하여 다른 지갑 찾기"
     },
+    "market_chart_loading": {
+        "es": "Cargando historial de precios", "de": "Preisverlauf wird geladen",
+        "fr": "Chargement de l'historique des prix", "pt": "Carregando histórico de preços",
+        "ru": "Загрузка истории цен", "zh": "正在加载价格历史",
+        "ja": "価格履歴を読み込み中", "ko": "가격 기록 불러오는 중"
+    },
 }
 
 def main():