UX 审查 P2 修复：X轴标签密度、极端温度视觉强化

- AiCityTemperatureChart：X轴标签从每4个→每3个显示，maxTicksLimit 6→8
- globals.css：新增 temp-extreme-hot（≥40°C）和 temp-extreme-cold（≤-5°C）样式
- PanelSections：Hero 温度值自动应用极端温度 CSS 类（橙色辉光/蓝色辉光）
- 华氏度自动适配：≥104°F 为极热，≤23°F 为极冷

Tested: npx tsc --noEmit

frontend/components/dashboard/PanelSections.tsx

@@ -1,5 +1,13 @@
 "use client";
 
+function getTempExtremeClass(temp: number | null | undefined, symbol?: string | null) {
+  if (temp == null || !Number.isFinite(temp)) return "";
+  const isF = String(symbol || "").toUpperCase().includes("F");
+  if (isF ? temp >= 104 : temp >= 40) return "temp-extreme-hot";
+  if (isF ? temp <= 23 : temp <= -5) return "temp-extreme-cold";
+  return "";
+}
+
 import type { ChartConfiguration } from "chart.js";
 import clsx from "clsx";
 import { startTransition, useMemo } from "react";
@@ -65,7 +73,7 @@ export function HeroSummary() {
           {weatherIcon} {weatherText}
         </span>
       </div>
-      <div className="hero-temp">
+      <div className={`hero-temp ${getTempExtremeClass(current.temp, data.temp_symbol)}`}>
         <span className="hero-value">
           {current.temp != null ? current.temp.toFixed(1) : "--"}
         </span>