refactor: extract gametime package, replace fmt.Printf with zap logging

Move time utilities (TimeAdjusted, TimeMidnight, TimeWeekStart, TimeWeekNext,
TimeGameAbsolute) from channelserver into common/gametime to break the
inappropriate dependency where signserver, entranceserver, and api imported
the 38K-line channelserver package just for time functions.

Replace all fmt.Printf debug logging in sys_session.go and handlers_object.go
with structured zap logging for consistent observability.

common/gametime/gametime.go

@@ -0,0 +1,32 @@
+package gametime
+
+import (
+	"time"
+)
+
+func Adjusted() time.Time {
+	baseTime := time.Now().In(time.FixedZone("UTC+9", 9*60*60))
+	return time.Date(baseTime.Year(), baseTime.Month(), baseTime.Day(), baseTime.Hour(), baseTime.Minute(), baseTime.Second(), baseTime.Nanosecond(), baseTime.Location())
+}
+
+func Midnight() time.Time {
+	baseTime := time.Now().In(time.FixedZone("UTC+9", 9*60*60))
+	return time.Date(baseTime.Year(), baseTime.Month(), baseTime.Day(), 0, 0, 0, 0, baseTime.Location())
+}
+
+func WeekStart() time.Time {
+	midnight := Midnight()
+	offset := int(midnight.Weekday()) - int(time.Monday)
+	if offset < 0 {
+		offset += 7
+	}
+	return midnight.Add(-time.Duration(offset) * 24 * time.Hour)
+}
+
+func WeekNext() time.Time {
+	return WeekStart().Add(time.Hour * 24 * 7)
+}
+
+func GameAbsolute() uint32 {
+	return uint32((Adjusted().Unix() - 2160) % 5760)
+}