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

docs/reference/api/schemas.md

@@ -9358,6 +9358,20 @@ Restarts will only happen on weekdays in this list on weeks which line up with W
 | `old_password` | string | false    |              |             |
 | `password`     | string | true     |              |             |
 
+## codersdk.UpdateUserPreferenceSettingsRequest
+
+```json
+{
+  "task_notification_alert_dismissed": true
+}
+```
+
+### Properties
+
+| Name                                | Type    | Required | Restrictions | Description |
+|-------------------------------------|---------|----------|--------------|-------------|
+| `task_notification_alert_dismissed` | boolean | false    |              |             |
+
 ## codersdk.UpdateUserProfileRequest
 
 ```json
@@ -9846,6 +9860,20 @@ If the schedule is empty, the user will be updated to use the default schedule.|
 | `name`  | string | false    |              |             |
 | `value` | string | false    |              |             |
 
+## codersdk.UserPreferenceSettings
+
+```json
+{
+  "task_notification_alert_dismissed": true
+}
+```
+
+### Properties
+
+| Name                                | Type    | Required | Restrictions | Description |
+|-------------------------------------|---------|----------|--------------|-------------|
+| `task_notification_alert_dismissed` | boolean | false    |              |             |
+
 ## codersdk.UserQuietHoursScheduleConfig
 
 ```json

docs/reference/api/users.md

@@ -1241,6 +1241,90 @@ curl -X PUT http://coder-server:8080/api/v2/users/{user}/password \
 
 To perform this operation, you must be authenticated. [Learn more](authentication.md).
 
+## Get user preference settings
+
+### Code samples
+
+```shell
+# Example request using curl
+curl -X GET http://coder-server:8080/api/v2/users/{user}/preferences \
+  -H 'Accept: application/json' \
+  -H 'Coder-Session-Token: API_KEY'
+```
+
+`GET /users/{user}/preferences`
+
+### Parameters
+
+| Name   | In   | Type   | Required | Description          |
+|--------|------|--------|----------|----------------------|
+| `user` | path | string | true     | User ID, name, or me |
+
+### Example responses
+
+> 200 Response
+
+```json
+{
+  "task_notification_alert_dismissed": true
+}
+```
+
+### Responses
+
+| Status | Meaning                                                 | Description | Schema                                                                       |
+|--------|---------------------------------------------------------|-------------|------------------------------------------------------------------------------|
+| 200    | [OK](https://tools.ietf.org/html/rfc7231#section-6.3.1) | OK          | [codersdk.UserPreferenceSettings](schemas.md#codersdkuserpreferencesettings) |
+
+To perform this operation, you must be authenticated. [Learn more](authentication.md).
+
+## Update user preference settings
+
+### Code samples
+
+```shell
+# Example request using curl
+curl -X PUT http://coder-server:8080/api/v2/users/{user}/preferences \
+  -H 'Content-Type: application/json' \
+  -H 'Accept: application/json' \
+  -H 'Coder-Session-Token: API_KEY'
+```
+
+`PUT /users/{user}/preferences`
+
+> Body parameter
+
+```json
+{
+  "task_notification_alert_dismissed": true
+}
+```
+
+### Parameters
+
+| Name   | In   | Type                                                                                                   | Required | Description             |
+|--------|------|--------------------------------------------------------------------------------------------------------|----------|-------------------------|
+| `user` | path | string                                                                                                 | true     | User ID, name, or me    |
+| `body` | body | [codersdk.UpdateUserPreferenceSettingsRequest](schemas.md#codersdkupdateuserpreferencesettingsrequest) | true     | New preference settings |
+
+### Example responses
+
+> 200 Response
+
+```json
+{
+  "task_notification_alert_dismissed": true
+}
+```
+
+### Responses
+
+| Status | Meaning                                                 | Description | Schema                                                                       |
+|--------|---------------------------------------------------------|-------------|------------------------------------------------------------------------------|
+| 200    | [OK](https://tools.ietf.org/html/rfc7231#section-6.3.1) | OK          | [codersdk.UserPreferenceSettings](schemas.md#codersdkuserpreferencesettings) |
+
+To perform this operation, you must be authenticated. [Learn more](authentication.md).
+
 ## Update user profile
 
 ### Code samples