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/handlers/task_handler.go

@@ -36,6 +36,12 @@ func (h *TaskHandler) ListTasks(c echo.Context) error {
 	user := c.Get(middleware.AuthUserKey).(*models.User)
 	userNow := middleware.GetUserNow(c)
 
+	// Auto-capture timezone from header for background job calculations (e.g., daily digest)
+	// This runs in a goroutine to avoid blocking the response
+	if tzHeader := c.Request().Header.Get("X-Timezone"); tzHeader != "" {
+		go h.taskService.UpdateUserTimezone(user.ID, tzHeader)
+	}
+
 	response, err := h.taskService.ListTasks(user.ID, userNow)
 	if err != nil {
 		return err