扩展模型区间端点至首尔和釜山，并做前端小幅清理

- GET /api/cities/model-range 新增 seoul/busan，总计 9 城
- 移除未使用的 react-leaflet 依赖
- 提取 chart Tooltip contentStyle 为共享常量 CHART_TOOLTIP_STYLE，消除 6 个文件中 15 处重复内联样式

Tested: npx tsc --noEmit pass
Confidence: high

frontend/components/ops/health/HealthPageClient.tsx

@@ -4,6 +4,7 @@ import { useEffect, useState } from "react";
 import { RefreshCcw, CheckCircle2, XCircle, AlertTriangle } from "lucide-react";
 import { Card, CardContent } from "@/components/ui/card";
 import { Button } from "@/components/ui/button";
+import { CHART_TOOLTIP_STYLE } from "@/lib/chart-utils";
 import {
   BarChart, Bar, XAxis, YAxis, CartesianGrid, Tooltip,
   ResponsiveContainer, Cell,
@@ -109,7 +110,7 @@ export function HealthPageClient() {
                 <XAxis type="number" stroke="rgba(255,255,255,0.2)" tick={{ fill: "#64748b", fontSize: 10 }} />
                 <YAxis type="category" dataKey="name" stroke="rgba(255,255,255,0.2)" tick={{ fill: "#94a3b8", fontSize: 11 }} width={120} />
                 <Tooltip
-                  contentStyle={{ background: "#1e293b", border: "1px solid rgba(255,255,255,0.1)", borderRadius: 8, color: "#e2e8f0", fontSize: 12 }}
+                  contentStyle={CHART_TOOLTIP_STYLE}
                   formatter={(value) => [`${value} ms`, "延迟"]}
                 />
                 <Bar dataKey="latency" radius={[0, 4, 4, 0]}>