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/users.sql

@@ -168,6 +168,29 @@ WHERE user_configs.user_id = @user_id
 	AND user_configs.key = 'terminal_font'
 RETURNING *;
 
+-- name: GetUserTaskNotificationAlertDismissed :one
+SELECT
+	value::boolean as task_notification_alert_dismissed
+FROM
+	user_configs
+WHERE
+	user_id = @user_id
+	AND key = 'preference_task_notification_alert_dismissed';
+
+-- name: UpdateUserTaskNotificationAlertDismissed :one
+INSERT INTO
+	user_configs (user_id, key, value)
+VALUES
+	(@user_id, 'preference_task_notification_alert_dismissed', (@task_notification_alert_dismissed::boolean)::text)
+ON CONFLICT
+	ON CONSTRAINT user_configs_pkey
+DO UPDATE
+SET
+	value = @task_notification_alert_dismissed
+WHERE user_configs.user_id = @user_id
+	AND user_configs.key = 'preference_task_notification_alert_dismissed'
+RETURNING value::boolean AS task_notification_alert_dismissed;
+
 -- name: UpdateUserRoles :one
 UPDATE
 	users