feat(ui): 设置页重做 + 同步页简化 + 评分依据页

需求 2.5 / 2.6 / 2.7 / 2.8 / 5.1 / 5.2 / 5.3

设置页
- 个人资料：身高/体重/出生日期/性别，另显示算出的 BMI 与年龄
- 单位：公制 / 英制
- 同步：自动同步开关、频率、历史范围
- 改动即存，不放「保存」按钮——每项都是单值且效果直观，
  而能保持脏状态的表单就是会悄悄丢编辑的表单
- 开关用原生 checkbox 只换绘制，键盘与读屏行为保持不变

/rating-basis/ 评分依据
- 11 项参考区间逐条列出边界与出处，包括「一般性参考，无权威标准」这种
  诚实的答案。判断从哪来必须能查到。

同步页
- 首要按钮「同步最新数据」，等几秒直接返回结果
- 次要按钮「同步历史」，按设置里的历史范围后台跑
- 状态压缩成三格：上次同步 / 自动同步 / 已同步天数
- 删掉原来四段说明文字，它们把两个动作埋在了下面

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

client/src/pages/RatingBasisPage.tsx

@@ -0,0 +1,73 @@
+import { useEffect, useState } from 'react';
+import { apiClient, errorMessage, RatingBasis } from '../services/api';
+import Screen from '../components/Screen';
+import Skeleton from '../components/Skeleton';
+import './BodyAge.css';
+
+/**
+ * Where every rating in the app comes from.
+ *
+ * A band that paints a number 偏低 is making a claim about the user's health.
+ * This screen is the receipt for that claim — one row per metric, with the
+ * source named, including the ones whose honest source is "general guidance,
+ * no authoritative standard".
+ */
+function RatingBasisPage() {
+  const [basis, setBasis] = useState<RatingBasis | null>(null);
+  const [loading, setLoading] = useState(true);
+  const [error, setError] = useState('');
+
+  useEffect(() => {
+    apiClient
+      .getRatingBasis()
+      .then(setBasis)
+      .catch((err) => setError(errorMessage(err, '加载失败')))
+      .finally(() => setLoading(false));
+  }, []);
+
+  if (loading) return <Screen title="评分依据" backLink><Skeleton count={5} /></Screen>;
+  if (error || !basis) {
+    return (
+      <Screen title="评分依据" backLink>
+        <div className="screen-error">{error || '加载失败'}</div>
+      </Screen>
+    );
+  }
+
+  return (
+    <Screen title="评分依据" backLink>
+      <p className="ba-summary">{basis.note}</p>
+
+      <section className="sec">
+        <h3 className="sec-title">各指标参考区间</h3>
+        <div className="ba-basis">
+          {basis.bands.map((b) => (
+            <div className="ba-basis-item" key={b.metric}>
+              <div className="ba-basis-name">{b.metric}</div>
+              <div className="ba-basis-detail rb-bands">{b.bands}</div>
+              <div className="ba-basis-source">来源：{b.source}</div>
+            </div>
+          ))}
+        </div>
+      </section>
+
+      <section className="sec">
+        <h3 className="sec-title">{basis.fitnessAge.title}</h3>
+        <p className="ba-summary">{basis.fitnessAge.summary}</p>
+        <div className="ba-basis">
+          {basis.fitnessAge.steps.map((s) => (
+            <div className="ba-basis-item" key={s.name}>
+              <div className="ba-basis-name">{s.name}</div>
+              <div className="ba-basis-detail">{s.detail}</div>
+              <div className="ba-basis-source">来源：{s.source}</div>
+            </div>
+          ))}
+        </div>
+      </section>
+
+      <p className="screen-disclaimer">{basis.fitnessAge.caveat}</p>
+    </Screen>
+  );
+}
+
+export default RatingBasisPage;