fix: 修复跨时区用户日期范围查询不准确的问题

问题：当用户时区与服务器时区不同时，日期范围查询使用服务器时区解析，
导致用户看到的数据与预期不符。

修复方案：
- 前端：所有 GET 请求自动携带用户时区参数
- 后端：新增时区辅助函数，所有日期解析和默认日期范围计算都使用用户时区
- 当用户时区为空或无效时，自动回退到服务器时区

🤖 Generated with [Claude Code](https://claude.ai/code)

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

backend/internal/pkg/timezone/timezone.go

@@ -122,3 +122,40 @@ func StartOfMonth(t time.Time) time.Time {
 func ParseInLocation(layout, value string) (time.Time, error) {
 	return time.ParseInLocation(layout, value, Location())
 }
+
+// ParseInUserLocation parses a time string in the user's timezone.
+// If userTZ is empty or invalid, falls back to the configured server timezone.
+func ParseInUserLocation(layout, value, userTZ string) (time.Time, error) {
+	loc := Location() // default to server timezone
+	if userTZ != "" {
+		if userLoc, err := time.LoadLocation(userTZ); err == nil {
+			loc = userLoc
+		}
+	}
+	return time.ParseInLocation(layout, value, loc)
+}
+
+// NowInUserLocation returns the current time in the user's timezone.
+// If userTZ is empty or invalid, falls back to the configured server timezone.
+func NowInUserLocation(userTZ string) time.Time {
+	if userTZ == "" {
+		return Now()
+	}
+	if userLoc, err := time.LoadLocation(userTZ); err == nil {
+		return time.Now().In(userLoc)
+	}
+	return Now()
+}
+
+// StartOfDayInUserLocation returns the start of the given day in the user's timezone.
+// If userTZ is empty or invalid, falls back to the configured server timezone.
+func StartOfDayInUserLocation(t time.Time, userTZ string) time.Time {
+	loc := Location()
+	if userTZ != "" {
+		if userLoc, err := time.LoadLocation(userTZ); err == nil {
+			loc = userLoc
+		}
+	}
+	t = t.In(loc)
+	return time.Date(t.Year(), t.Month(), t.Day(), 0, 0, 0, 0, loc)
+}