fix: 返回键失效的根因 + 同步页重做 + 连接池会永久阻塞线程

返回键
- 根因是 Framework7 的页面过渡由动画事件驱动：push 时把 allowPageChange
  置 false，等动画报告结束再恢复。这个报告不来，路由就永久卡住，之后每次
  导航都被静默丢弃，back() 还会把上一页重建一份而不是弹出。
  实测对照：navigate({animate:false}) 前后状态完全正确，带动画则必卡。
  因此关掉页面过渡动画——导航同步完成，处处正确。动效改由内容承担
  （卡片入场、hero 揭示、顶部进度条），这个取舍里正确性优先。
- Screen 的返回改为显式 handler，先清掉残留过渡状态再 back()，
  不依赖路由自己的闸门。注意只清视图上的 router-transition 类：
  页面自身的 page-previous 是 F7 判断「回到哪一页」的依据，
  一并清掉会导致重建出一个重复的页面（中途踩过这个坑）。

同步页
- .btn 系列样式原本只定义在 pages/Pages.css，而那个文件只被一个没有路由的
  遗留页面引用，所以真实页面上按钮全都退化成 Framework7 的默认样式——
  就是你看到的三条链接。样式移进每个界面都会加载的 Screen.css。
- 主次分明：一个填充主按钮 + 两个带副标题的次按钮；补上「同步会取哪些数据」
  说明，页面不再是一大片空白。
- 进度条显示当前阶段（每日数据 2026-08-01 / 运动详情 12/174 / 身体成分…），
  原来只有「0 / 730 天」，几分钟里完全看不出在做什么。

后端
- MariaDB 连接池：_mariadb_release 用的是阻塞 put()，而队列 maxsize=10，
  _mariadb_acquire 在池空时又会新建连接。并发超过 10 之后，归还的线程会
  永久停在 put() 上，请求就此挂死。改为 put_nowait，多出来的连接直接关闭。
- 进程重启会带走同步线程却留下 status=syncing 的行，界面上是一个永远不动
  的进度条，还拒绝开始新同步。启动时清理。

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

client/src/components/Screen.css

@@ -1,3 +1,47 @@
+/* Buttons -----------------------------------------------------------------
+   These lived in pages/Pages.css, which only one unrouted leftover page
+   imported — so on every real screen the .btn classes resolved to nothing and
+   Framework7's default button styling showed through instead. They belong
+   here, in the stylesheet every screen loads. */
+.btn {
+  padding: 0.7rem 1.1rem;
+  border: 1px solid transparent;
+  border-radius: 11px;
+  font-size: 0.92rem;
+  font-weight: 550;
+  font-family: inherit;
+  cursor: pointer;
+  text-decoration: none;
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  gap: 0.4rem;
+  width: auto;
+  transition: transform 0.15s var(--ease), filter 0.15s var(--ease),
+              border-color 0.15s var(--ease), background 0.15s var(--ease);
+}
+
+.btn:active:not(:disabled) { transform: scale(0.975); }
+.btn:disabled { opacity: 0.5; cursor: not-allowed; }
+
+.btn-primary { background: var(--accent-solid); color: #fff; }
+.btn-primary:hover:not(:disabled) { filter: brightness(1.08); }
+
+.btn-plain {
+  background: var(--surface-1);
+  border-color: var(--border);
+  color: var(--text-primary);
+}
+
+.btn-plain:hover:not(:disabled) { border-color: var(--border-strong); }
+
+.btn-large { width: 100%; padding: 0.85rem 1.5rem; font-size: 1rem; }
+
+@media (prefers-reduced-motion: reduce) {
+  .btn { transition: none; }
+  .btn:active:not(:disabled) { transform: none; }
+}
+
 /* Sections ---------------------------------------------------------------- */
 .sec {
   margin-bottom: 1.5rem;

client/src/components/Screen.tsx

@@ -1,5 +1,5 @@
 import { ReactNode, useEffect, useState } from 'react';
-import { Page, Navbar, NavRight, f7 } from 'framework7-react';
+import { Page, Navbar, NavLeft, NavRight, Link, f7 } from 'framework7-react';
 import { apiClient } from '../services/api';
 import './Screen.css';
 
@@ -16,6 +16,34 @@ interface ScreenProps {
   children: ReactNode;
 }
 
+/**
+ * Go back without depending on the router's own guard.
+ *
+ * Framework7 sets `router.allowPageChange = false` for the duration of a page
+ * transition and restores it when the transition reports it has ended. When
+ * that report never arrives the flag stays false and `router.back()` becomes a
+ * no-op — a back button that does nothing, with a reload as the only way out.
+ * Clearing the leftover transition state first makes the button work whether
+ * or not the router got stuck.
+ */
+function goBack() {
+  const router = f7.views.current?.router;
+  if (!router) return;
+
+  // Only the view's transition flags are cleared. The pages' own
+  // page-previous / page-next classes are how F7 knows which page to return
+  // to — stripping those makes `back()` rebuild the previous screen from
+  // scratch and stack a duplicate instead of popping.
+  const el = router.view?.el as HTMLElement | undefined;
+  el?.classList.remove(
+    'router-transition', 'router-transition-forward',
+    'router-transition-backward', 'router-transition-css-forward',
+    'router-transition-css-backward'
+  );
+  router.allowPageChange = true;
+  router.back();
+}
+
 /**
  * Common page chrome.
  *
@@ -44,7 +72,18 @@ function Screen({
 
   return (
     <Page>
-      <Navbar large={large} transparent={large} title={title} subtitle={subtitle} backLink={backLink ? '返回' : undefined}>
+      <Navbar large={large} transparent={large} title={title} subtitle={subtitle}>
+        {backLink && (
+          <NavLeft>
+            <Link
+              className="back-link"
+              onClick={goBack}
+              iconIos="f7:chevron_left"
+              iconMd="f7:chevron_left"
+              aria-label="返回"
+            />
+          </NavLeft>
+        )}
         {/* Rendered only when the screen actually has an action: an empty
             NavRight still paints a container, which showed up as a stray
             grey box beside the title. */}