fix: add toast error feedback for API failures in signals and data explorer pages

Replace silent console.error-only handling with user-visible toast notifications
using sonner, while keeping console.error for debugging.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/app/admin/data/explorer/page.tsx

@@ -7,6 +7,7 @@ import { Card, CardContent, CardHeader, CardTitle } from '@/components/ui/card';
 import { Button } from '@/components/ui/button';
 import { Input } from '@/components/ui/input';
 import { api } from '@/lib/api';
+import { toast } from 'sonner';
 
 type Tab = 'stocks' | 'etfs' | 'sectors' | 'valuations';
 
@@ -96,7 +97,9 @@ export default function DataExplorerPage() {
       const endpoint = `/api/data/${tab}?${params}`;
       const result = await api.get<PaginatedResponse<unknown>>(endpoint);
       setData(result);
-    } catch {
+    } catch (err) {
+      console.error('Failed to fetch data:', err);
+      toast.error('데이터를 불러오는데 실패했습니다.');
       setData(null);
     } finally {
       setFetching(false);
@@ -128,7 +131,9 @@ export default function DataExplorerPage() {
         : `/api/data/etfs/${ticker}/prices`;
       const result = await api.get<PricePoint[]>(endpoint);
       setPrices(result);
-    } catch {
+    } catch (err) {
+      console.error('Failed to fetch prices:', err);
+      toast.error('가격 데이터를 불러오는데 실패했습니다.');
       setPrices([]);
     } finally {
       setPriceLoading(false);

frontend/src/app/signals/page.tsx

@@ -25,6 +25,7 @@ import {
   SelectValue,
 } from '@/components/ui/select';
 import { api } from '@/lib/api';
+import { toast } from 'sonner';
 import { Radio, History, RefreshCw, ArrowUpCircle, ArrowDownCircle, MinusCircle, Play } from 'lucide-react';
 
 interface Signal {
@@ -159,6 +160,7 @@ export default function SignalsPage() {
       setTodaySignals(data);
     } catch (err) {
       console.error('Failed to fetch today signals:', err);
+      toast.error('오늘의 신호를 불러오는데 실패했습니다.');
     }
   };
 
@@ -174,6 +176,7 @@ export default function SignalsPage() {
       setHistorySignals(data);
     } catch (err) {
       console.error('Failed to fetch signal history:', err);
+      toast.error('신호 이력을 불러오는데 실패했습니다.');
     }
   };
 
@@ -183,6 +186,7 @@ export default function SignalsPage() {
       setPortfolios(data);
     } catch (err) {
       console.error('Failed to fetch portfolios:', err);
+      toast.error('포트폴리오 목록을 불러오는데 실패했습니다.');
     }
   };
 
@@ -250,8 +254,9 @@ export default function SignalsPage() {
               if (ps.recommended_quantity > 0) {
                 setExecuteQuantity(String(ps.recommended_quantity));
               }
-            } catch {
-              // Position sizing is optional
+            } catch (err) {
+              console.error('Failed to fetch position sizing:', err);
+              toast.error('포지션 사이징 정보를 불러오는데 실패했습니다.');
             }
           }
         }