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/coderd.go

@@ -1336,6 +1336,8 @@ func New(options *Options) *API {
 						})
 						r.Get("/appearance", api.userAppearanceSettings)
 						r.Put("/appearance", api.putUserAppearanceSettings)
+						r.Get("/preferences", api.userPreferenceSettings)
+						r.Put("/preferences", api.putUserPreferenceSettings)
 						r.Route("/password", func(r chi.Router) {
 							r.Use(httpmw.RateLimit(options.LoginRateLimit, time.Minute))
 							r.Put("/", api.putUserPassword)