feat: 后台管理 Admin 仪表盘

新增完整的后台管理系统 (/admin):
- Dashboard: 系统概览、最近采集状态、预测统计
- Collection: 数据采集触发(bzzoiro/understat/injuries)
- Predictions: 预测历史查看、触发新预测
- Backtest: 回测配置与结果查看
- Monitoring: 系统健康、错误日志、死色队列
- Config: API Key 与数据源配置

技术栈: React Router + Tailwind 暗色主题 + TypeScript
文件: 11 个新文件, +21KB JS / +5KB CSS

frontend/src/index.css

@@ -100,3 +100,54 @@
     @apply animate-pulse rounded-none bg-ink-200;
   }
 }
+
+/* ── Admin 后台:暗色主题覆盖 ── */
+@layer base {
+  /* Admin 区域内滚动条暗色化 */
+  .dark-scroll::-webkit-scrollbar-thumb {
+    @apply bg-gray-600;
+  }
+  .dark-scroll::-webkit-scrollbar-thumb:hover {
+    @apply bg-gray-500;
+  }
+}
+
+@layer components {
+  /* Admin 输入框统一样式 */
+  .admin-input {
+    @apply w-full rounded-md border border-gray-700 bg-gray-800 px-3 py-2 text-sm text-gray-200
+           transition-colors placeholder:text-gray-600
+           focus:border-blue-500 focus:outline-none focus:ring-1 focus:ring-blue-500;
+  }
+
+  /* Admin 按钮 */
+  .admin-btn {
+    @apply inline-flex items-center justify-center gap-1.5 rounded-md border border-gray-700
+           bg-gray-800 px-4 py-2 text-sm font-medium text-gray-200
+           transition-colors hover:border-gray-600 hover:bg-gray-700
+           disabled:cursor-not-allowed disabled:opacity-40;
+  }
+
+  .admin-btn-primary {
+    @apply border-blue-600 bg-blue-600 text-white hover:bg-blue-700 hover:border-blue-700;
+  }
+
+  .admin-btn-danger {
+    @apply border-red-600 bg-red-600 text-white hover:bg-red-700 hover:border-red-700;
+  }
+
+  /* Admin 卡片通用 */
+  .admin-card {
+    @apply rounded-lg border border-gray-800 bg-gray-900;
+  }
+
+  /* 表格行动态 */
+  .admin-row-hover {
+    @apply transition-colors hover:bg-gray-800/40;
+  }
+
+  /* 状态点 */
+  .status-dot {
+    @apply inline-block h-2 w-2 rounded-full;
+  }
+}