Add timezone-aware daily digest notifications

The daily digest notification count was inconsistent with the kanban UI
because the server used UTC time while the client used local time.
A task due Dec 24 would appear overdue on the server (UTC Dec 25) but
still show as "due today" for the user (local Dec 24).

Changes:
- Add timezone column to notification_preference table
- Auto-capture user's timezone from X-Timezone header when fetching tasks
- Use stored timezone in HandleDailyDigest for accurate overdue calculation

The mobile app already sends X-Timezone on every request, so no client
changes are needed. The timezone is captured on each app launch when
the tasks API is called.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/services/task_service.go

@@ -996,3 +996,14 @@ func (s *TaskService) GetFrequencies() ([]responses.TaskFrequencyResponse, error
 	}
 	return result, nil
 }
+
+// === Timezone ===
+
+// UpdateUserTimezone updates the user's timezone for background job calculations.
+// This is called from handlers when the X-Timezone header is present.
+// Delegates to NotificationService since timezone is stored in notification preferences.
+func (s *TaskService) UpdateUserTimezone(userID uint, timezone string) {
+	if s.notificationService != nil {
+		s.notificationService.UpdateUserTimezone(userID, timezone)
+	}
+}