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/querier.go

@@ -447,6 +447,7 @@ type sqlcQuerier interface {
 	// We do not start counting from 0 at the start_time. We check the last status change before the start_time for each user. As such,
 	// the result shows the total number of users in each status on any particular day.
 	GetUserStatusCounts(ctx context.Context, arg GetUserStatusCountsParams) ([]GetUserStatusCountsRow, error)
+	GetUserTaskNotificationAlertDismissed(ctx context.Context, userID uuid.UUID) (bool, error)
 	GetUserTerminalFont(ctx context.Context, userID uuid.UUID) (string, error)
 	GetUserThemePreference(ctx context.Context, userID uuid.UUID) (string, error)
 	GetUserWorkspaceBuildParameters(ctx context.Context, arg GetUserWorkspaceBuildParametersParams) ([]GetUserWorkspaceBuildParametersRow, error)
@@ -714,6 +715,7 @@ type sqlcQuerier interface {
 	UpdateUserRoles(ctx context.Context, arg UpdateUserRolesParams) (User, error)
 	UpdateUserSecret(ctx context.Context, arg UpdateUserSecretParams) (UserSecret, error)
 	UpdateUserStatus(ctx context.Context, arg UpdateUserStatusParams) (User, error)
+	UpdateUserTaskNotificationAlertDismissed(ctx context.Context, arg UpdateUserTaskNotificationAlertDismissedParams) (bool, error)
 	UpdateUserTerminalFont(ctx context.Context, arg UpdateUserTerminalFontParams) (UserConfig, error)
 	UpdateUserThemePreference(ctx context.Context, arg UpdateUserThemePreferenceParams) (UserConfig, error)
 	UpdateVolumeResourceMonitor(ctx context.Context, arg UpdateVolumeResourceMonitorParams) error