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>

backend/services/garmin.py

@@ -68,9 +68,23 @@ def get_sync_status(user_id):
         "progressCurrent": row.get("progress_current"),
         "progressTotal": row.get("progress_total"),
         "startedAt": row.get("started_at"),
+        "stage": row.get("stage"),
     }
 
 
+def reset_stale_syncs():
+    """Clear a "syncing" status left behind by a process that went away.
+
+    The status lives in the database but the work lives in a thread. A restart
+    (or a crash) takes the thread and leaves the row, so the UI shows a
+    progress bar that will never move and refuses to start a new sync.
+    """
+    execute(
+        "UPDATE sync_status SET status = 'idle', stage = NULL "
+        "WHERE status = 'syncing'"
+    )
+
+
 class MFARequired(RuntimeError):
     """Raised when a password login needs a code this process cannot obtain."""
 
@@ -761,6 +775,7 @@ def sync_data(user_id, creds, days=None, client=None):
     _set_sync_status(
         user_id, "syncing", now,
         records_synced=0, progress_current=0, progress_total=days,
+        stage="连接 Garmin",
     )
 
     try:
@@ -809,10 +824,13 @@ def sync_data(user_id, creds, days=None, client=None):
             _set_sync_status(
                 user_id, "syncing", now,
                 records_synced=days_synced, progress_current=i + 1,
-                progress_total=days,
+                progress_total=days, stage=f"每日数据 {date_str}",
             )
 
     activities_synced = 0
+    _set_sync_status(user_id, "syncing", now, records_synced=days_synced,
+                     progress_current=days, progress_total=days,
+                     stage="运动记录")
     try:
         activities_synced = _sync_activities(
             client, user_id, start_date, today.isoformat()
@@ -824,13 +842,23 @@ def sync_data(user_id, creds, days=None, client=None):
     # opening one later is a local read.
     details_synced = 0
     try:
-        details_synced = sync_activity_details(client, user_id)
+        details_synced = sync_activity_details(
+            client, user_id,
+            on_progress=lambda d, n: _set_sync_status(
+                user_id, "syncing", now, records_synced=days_synced,
+                progress_current=days, progress_total=days,
+                stage=f"运动详情 {d}/{n}"),
+        )
     except Exception as e:
         day_errors.append(f"activity_details: {describe(e)}")
 
     # Everything else Garmin holds: body composition, blood pressure, race
     # predictions, challenges and devices. Account-wide, so once per sync.
     extra_counts = {}
+    stage_names = {
+        "bodyComposition": "身体成分", "bloodPressure": "血压",
+        "racePredictions": "成绩预测", "challenges": "挑战赛", "devices": "设备",
+    }
     for name, call in (
         ("bodyComposition",
          lambda: extras.sync_body_composition(client, user_id, start_date,
@@ -843,6 +871,9 @@ def sync_data(user_id, creds, days=None, client=None):
         ("challenges", lambda: extras.sync_challenges(client, user_id)),
         ("devices", lambda: extras.sync_devices(client, user_id)),
     ):
+        _set_sync_status(user_id, "syncing", now, records_synced=days_synced,
+                         progress_current=days, progress_total=days,
+                         stage=stage_names.get(name, name))
         try:
             extra_counts[name] = call()
         except Exception as e:  # noqa: BLE001 - one section must not fail the sync
@@ -871,7 +902,7 @@ def sync_data(user_id, creds, days=None, client=None):
 
     _set_sync_status(
         user_id, "idle", now, records_synced=days_synced,
-        progress_current=days, progress_total=days,
+        progress_current=days, progress_total=days, stage=None,
         last_error="; ".join(day_errors[:3]) if day_errors else None,
     )
     message = (