feat: 仪表盘+催办在休息日跳过填报检查

- 新增 is_working_day() 工具函数，自动跳过周末 + 可配置法定假日
- 仪表盘填报进度新增 is_rest_day 字段，休息日显示灰「休」印章
- 催办调度改用 is_working_day() 替代原始 weekday 检查
- 系统设置新增「法定节假日」配置卡片，支局长可管理假日日期

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

backend/app/utils/timezone.py

@@ -11,3 +11,21 @@ def today_cst() -> date:
 def parse_date(s: str) -> date:
     """Parse a date string that may be YYYY-MM-DD or a full ISO datetime."""
     return date.fromisoformat(s[:10])
+
+
+def is_working_day(d: date | None = None, holidays: set[date] | None = None) -> bool:
+    """Check if a date is a Chinese working day.
+
+    Returns False for:
+    - Saturdays and Sundays (weekday >= 5)
+    - Dates in the holidays set (configurable via system_config)
+
+    Note: makeup workdays (调休, where Saturday/Sunday becomes a workday)
+    are not currently handled. If needed, add a 'workdays' config set.
+    """
+    d = d or today_cst()
+    if d.weekday() >= 5:  # Saturday or Sunday
+        return False
+    if holidays and d in holidays:
+        return False
+    return True