feat(garmin): 增加「退出 Garmin 账号」功能，删除已保存令牌以便重新登录

backend: services/garmin.py 新增 delete_token()，删除 garmin_tokens 行并 forget_client 丢弃缓存会话；routes/garmin.py 新增 POST /api/garmin/disconnect（require_auth），返回 ok + 提示文案。
frontend: api.ts 增加 disconnectGarmin()；SyncPage 增加「退出 Garmin 账号」按钮（带二次确认弹窗）；DataSync.css 增加危险色样式。
设计：仅删除 OAuth 令牌，保留 users.garmin_email，下次重登只需密码；已同步的健康数据不受影响。

client/src/pages/DataSync.css

@@ -281,3 +281,21 @@
 }
 
 .sync-list li { margin-bottom: 0.2rem; }
+
+/* Account actions — destructive ones are flagged in red so they don't read
+   like the routine sync buttons above them. */
+.sync-account {
+  margin-top: 1.4rem;
+  padding-top: 1.1rem;
+  border-top: 1px solid var(--border);
+}
+
+.sync-disconnect {
+  color: var(--danger, #d23b3b);
+  border-color: var(--danger, #d23b3b);
+}
+
+.sync-disconnect:hover:not(:disabled) {
+  background: var(--danger, #d23b3b);
+  color: #fff;
+}

client/src/pages/SyncPage.tsx

@@ -164,6 +164,30 @@ function SyncPage() {
     setCodeSubmitted(false);
   };
 
+  // --- account ------------------------------------------------------------
+  /** Delete the stored Garmin token so the next sync forces a fresh login. */
+  const disconnect = async () => {
+    if (
+      !window.confirm(
+        '退出 Garmin 账号会删除已保存的授权令牌，下次同步需重新登录。确定继续？'
+      )
+    ) {
+      return;
+    }
+    setError('');
+    setMessage('');
+    setLoading(true);
+    try {
+      const res = await apiClient.disconnectGarmin();
+      setHasToken(false);
+      setMessage(res.message || '已退出 Garmin 账号，下次同步需重新登录获取新令牌。');
+    } catch (err: any) {
+      setError(errorMessage(err, '退出失败'));
+    } finally {
+      setLoading(false);
+    }
+  };
+
   // --- sync ---------------------------------------------------------------
 
   /** The last couple of days, awaited inline — seconds, not minutes. */
@@ -382,6 +406,19 @@ function SyncPage() {
                   </span>
                 </button>
               </div>
+
+              <div className="sync-account">
+                <button
+                  className="btn btn-plain sync-disconnect"
+                  onClick={disconnect}
+                  disabled={busy}
+                >
+                  退出 Garmin 账号
+                </button>
+                <p className="field-hint">
+                  删除已保存的授权令牌，下次同步需重新登录获取新令牌；已同步的数据不会丢失。
+                </p>
+              </div>
             </>
           )}
 

client/src/services/api.ts

@@ -467,6 +467,14 @@ class ApiClient {
     return data.hasToken;
   }
 
+  /** Drop the stored Garmin OAuth token; the next sync must re-authenticate. */
+  async disconnectGarmin() {
+    const { data } = await this.client.post<{ ok: boolean; message?: string }>(
+      '/garmin/disconnect'
+    );
+    return data;
+  }
+
   /**
    * Start an interactive Garmin login. Returns a session id; the login runs
    * in the background and parks if Garmin asks for a two-factor code.