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/dbauthz/dbauthz.go

@@ -3431,6 +3431,17 @@ func (q *querier) GetUserStatusCounts(ctx context.Context, arg database.GetUserS
 	return q.db.GetUserStatusCounts(ctx, arg)
 }
 
+func (q *querier) GetUserTaskNotificationAlertDismissed(ctx context.Context, userID uuid.UUID) (bool, error) {
+	user, err := q.db.GetUserByID(ctx, userID)
+	if err != nil {
+		return false, err
+	}
+	if err := q.authorizeContext(ctx, policy.ActionReadPersonal, user); err != nil {
+		return false, err
+	}
+	return q.db.GetUserTaskNotificationAlertDismissed(ctx, userID)
+}
+
 func (q *querier) GetUserTerminalFont(ctx context.Context, userID uuid.UUID) (string, error) {
 	u, err := q.db.GetUserByID(ctx, userID)
 	if err != nil {
@@ -5464,6 +5475,17 @@ func (q *querier) UpdateUserStatus(ctx context.Context, arg database.UpdateUserS
 	return updateWithReturn(q.log, q.auth, fetch, q.db.UpdateUserStatus)(ctx, arg)
 }
 
+func (q *querier) UpdateUserTaskNotificationAlertDismissed(ctx context.Context, arg database.UpdateUserTaskNotificationAlertDismissedParams) (bool, error) {
+	user, err := q.db.GetUserByID(ctx, arg.UserID)
+	if err != nil {
+		return false, err
+	}
+	if err := q.authorizeContext(ctx, policy.ActionUpdatePersonal, user); err != nil {
+		return false, err
+	}
+	return q.db.UpdateUserTaskNotificationAlertDismissed(ctx, arg)
+}
+
 func (q *querier) UpdateUserTerminalFont(ctx context.Context, arg database.UpdateUserTerminalFontParams) (database.UserConfig, error) {
 	u, err := q.db.GetUserByID(ctx, arg.UserID)
 	if err != nil {

coderd/database/dbauthz/dbauthz_test.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"net"
 	"reflect"
+	"strconv"
 	"testing"
 	"time"
 
@@ -1477,6 +1478,21 @@ func (s *MethodTestSuite) TestUser() {
 		dbm.EXPECT().UpdateUserTerminalFont(gomock.Any(), arg).Return(uc, nil).AnyTimes()
 		check.Args(arg).Asserts(u, policy.ActionUpdatePersonal).Returns(uc)
 	}))
+	s.Run("GetUserTaskNotificationAlertDismissed", s.Mocked(func(dbm *dbmock.MockStore, faker *gofakeit.Faker, check *expects) {
+		u := testutil.Fake(s.T(), faker, database.User{})
+		dbm.EXPECT().GetUserByID(gomock.Any(), u.ID).Return(u, nil).AnyTimes()
+		dbm.EXPECT().GetUserTaskNotificationAlertDismissed(gomock.Any(), u.ID).Return(false, nil).AnyTimes()
+		check.Args(u.ID).Asserts(u, policy.ActionReadPersonal).Returns(false)
+	}))
+	s.Run("UpdateUserTaskNotificationAlertDismissed", s.Mocked(func(dbm *dbmock.MockStore, faker *gofakeit.Faker, check *expects) {
+		user := testutil.Fake(s.T(), faker, database.User{})
+		userConfig := database.UserConfig{UserID: user.ID, Key: "task_notification_alert_dismissed", Value: "false"}
+		userConfigValue, _ := strconv.ParseBool(userConfig.Value)
+		arg := database.UpdateUserTaskNotificationAlertDismissedParams{UserID: user.ID, TaskNotificationAlertDismissed: userConfigValue}
+		dbm.EXPECT().GetUserByID(gomock.Any(), user.ID).Return(user, nil).AnyTimes()
+		dbm.EXPECT().UpdateUserTaskNotificationAlertDismissed(gomock.Any(), arg).Return(false, nil).AnyTimes()
+		check.Args(arg).Asserts(user, policy.ActionUpdatePersonal).Returns(userConfigValue)
+	}))
 	s.Run("UpdateUserStatus", s.Mocked(func(dbm *dbmock.MockStore, faker *gofakeit.Faker, check *expects) {
 		u := testutil.Fake(s.T(), faker, database.User{})
 		arg := database.UpdateUserStatusParams{ID: u.ID, Status: u.Status, UpdatedAt: u.UpdatedAt}