feat: add notification warning alert to Tasks page (#20900)

## Problem

Users may not realize that task notifications are disabled by default.
To improve awareness, we show a warning alert on the Tasks page when all
task notifications are disabled.

**Alert visibility logic:**
- Shows when **all** task notification templates (Task Working, Task
Idle, Task Completed, Task Failed) are disabled
- Can be dismissed by the user, which stores the dismissal in the user
preferences API
- If the user later enables any task notification in Account Settings,
the dismissal state is cleared so the alert will show again if they
disable all notifications in the future

<img width="2980" height="1588" alt="Screenshot 2025-11-25 at 17 48 17"
src="https://github.com/user-attachments/assets/316bf097-d9d2-4489-bc16-2987ba45f45c"
/>

## Changes

- Added a warning alert to the Tasks page when all task notifications
are disabled
- Introduced new `/users/{user}/preferences` endpoint to manage user
preferences (stored in `user_configs` table)
- Alert is dismissible and stores the dismissal state via the new user
preferences API endpoint
- Enabling any task notification in Account Settings clears the
dismissal state via the preferences API
- Added comprehensive Storybook stories for both TasksPage and
NotificationsPage to test all alert visibility states and interactions

Closes: https://github.com/coder/internal/issues/1089

coderd/database/queries.sql.go

@@ -16324,6 +16324,23 @@ func (q *sqlQuerier) GetUserCount(ctx context.Context, includeSystem bool) (int6
 	return count, err
 }
 
+const getUserTaskNotificationAlertDismissed = `-- name: GetUserTaskNotificationAlertDismissed :one
+SELECT
+	value::boolean as task_notification_alert_dismissed
+FROM
+	user_configs
+WHERE
+	user_id = $1
+	AND key = 'preference_task_notification_alert_dismissed'
+`
+
+func (q *sqlQuerier) GetUserTaskNotificationAlertDismissed(ctx context.Context, userID uuid.UUID) (bool, error) {
+	row := q.db.QueryRowContext(ctx, getUserTaskNotificationAlertDismissed, userID)
+	var task_notification_alert_dismissed bool
+	err := row.Scan(&task_notification_alert_dismissed)
+	return task_notification_alert_dismissed, err
+}
+
 const getUserTerminalFont = `-- name: GetUserTerminalFont :one
 SELECT
 	value as terminal_font
@@ -17076,6 +17093,33 @@ func (q *sqlQuerier) UpdateUserStatus(ctx context.Context, arg UpdateUserStatusP
 	return i, err
 }
 
+const updateUserTaskNotificationAlertDismissed = `-- name: UpdateUserTaskNotificationAlertDismissed :one
+INSERT INTO
+	user_configs (user_id, key, value)
+VALUES
+	($1, 'preference_task_notification_alert_dismissed', ($2::boolean)::text)
+ON CONFLICT
+	ON CONSTRAINT user_configs_pkey
+DO UPDATE
+SET
+	value = $2
+WHERE user_configs.user_id = $1
+	AND user_configs.key = 'preference_task_notification_alert_dismissed'
+RETURNING value::boolean AS task_notification_alert_dismissed
+`
+
+type UpdateUserTaskNotificationAlertDismissedParams struct {
+	UserID                         uuid.UUID `db:"user_id" json:"user_id"`
+	TaskNotificationAlertDismissed bool      `db:"task_notification_alert_dismissed" json:"task_notification_alert_dismissed"`
+}
+
+func (q *sqlQuerier) UpdateUserTaskNotificationAlertDismissed(ctx context.Context, arg UpdateUserTaskNotificationAlertDismissedParams) (bool, error) {
+	row := q.db.QueryRowContext(ctx, updateUserTaskNotificationAlertDismissed, arg.UserID, arg.TaskNotificationAlertDismissed)
+	var task_notification_alert_dismissed bool
+	err := row.Scan(&task_notification_alert_dismissed)
+	return task_notification_alert_dismissed, err
+}
+
 const updateUserTerminalFont = `-- name: UpdateUserTerminalFont :one
 INSERT INTO
 	user_configs (user_id, key, value)