build(infra): KIS API 클라이언트 시그니처 일원화 및 빌드 수정

- 수정: KisApiClient 메서드 반환 타입 Task<string> → Task<Dictionary<string, object>>로 통일
- 수정: GetOrRefreshTokenAsync ReadAsAsync → ReadFromJsonAsync로 변경
- 수정: PlaceholderKisApiClient IKisApiClient 인터페이스 완전 구현
- 수정: CollectionEndpoints 모든 WithOpenApi() 호출 제거
- 수정: Program.cs using 지시문 순서 재정렬
- 추가: Client/_Imports.razor Fluent UI 컴포넌트 네임스페이스 정의

이제 빌드 성공: QuantEngine.Web 프로젝트 컴파일 완료 (경고 0, 에러 0)

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/dotnet/QuantEngine.Web/Infrastructure/PlaceholderImplementations.cs

@@ -107,19 +107,58 @@ public class PlaceholderTokenCache : ITokenCache
 
 public class PlaceholderKisApiClient : IKisApiClient
 {
-    // Placeholder: To be implemented with actual KIS API calls
-    public Task<string?> GetAccessTokenAsync(string account = "mock")
+    public Task<Dictionary<string, object>> GetCurrentPriceAsync(string code, string account = "mock")
     {
-        return Task.FromResult<string?>("placeholder_token");
+        return Task.FromResult(new Dictionary<string, object>
+        {
+            { "code", code },
+            { "price", 0 },
+            { "change", 0 },
+            { "changeRate", 0 }
+        });
     }
 
-    public Task<dynamic?> GetQuotationAsync(string ticker, string account = "mock")
+    public Task<Dictionary<string, object>> GetAskingPrice10LevelAsync(string code, string account = "mock")
     {
-        return Task.FromResult<dynamic?>(null);
+        return Task.FromResult(new Dictionary<string, object>
+        {
+            { "code", code },
+            { "askLevels", new List<object>() },
+            { "bidLevels", new List<object>() }
+        });
     }
 
-    public Task<dynamic?> GetRankingAsync(string sort = "price", int limit = 10, string account = "mock")
+    public Task<Dictionary<string, object>> GetDailyShortSaleAsync(string code, string startDate, string endDate, string account = "mock")
     {
-        return Task.FromResult<dynamic?>(null);
+        return Task.FromResult(new Dictionary<string, object>
+        {
+            { "code", code },
+            { "startDate", startDate },
+            { "endDate", endDate },
+            { "data", new List<object>() }
+        });
+    }
+
+    public Task<Dictionary<string, object>> GetDailyItemChartPriceAsync(string code, string startDate, string endDate, string period = "D", string account = "mock")
+    {
+        return Task.FromResult(new Dictionary<string, object>
+        {
+            { "code", code },
+            { "startDate", startDate },
+            { "endDate", endDate },
+            { "period", period },
+            { "candles", new List<object>() }
+        });
+    }
+
+    public Task<Dictionary<string, object>> GetInvestorTrendAsync(string code, string account = "mock")
+    {
+        return Task.FromResult(new Dictionary<string, object>
+        {
+            { "code", code },
+            { "individual", 0 },
+            { "foreign", 0 },
+            { "institution", 0 }
+        });
     }
 }