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/users.go

@@ -1077,6 +1077,74 @@ func (api *API) putUserAppearanceSettings(rw http.ResponseWriter, r *http.Reques
 	})
 }
 
+// @Summary Get user preference settings
+// @ID get-user-preference-settings
+// @Security CoderSessionToken
+// @Produce json
+// @Tags Users
+// @Param user path string true "User ID, name, or me"
+// @Success 200 {object} codersdk.UserPreferenceSettings
+// @Router /users/{user}/preferences [get]
+func (api *API) userPreferenceSettings(rw http.ResponseWriter, r *http.Request) {
+	var (
+		ctx  = r.Context()
+		user = httpmw.UserParam(r)
+	)
+
+	taskAlertDismissed, err := api.Database.GetUserTaskNotificationAlertDismissed(ctx, user.ID)
+	if err != nil {
+		if !errors.Is(err, sql.ErrNoRows) {
+			httpapi.Write(ctx, rw, http.StatusInternalServerError, codersdk.Response{
+				Message: "Error reading user preference settings.",
+				Detail:  err.Error(),
+			})
+			return
+		}
+	}
+
+	httpapi.Write(ctx, rw, http.StatusOK, codersdk.UserPreferenceSettings{
+		TaskNotificationAlertDismissed: taskAlertDismissed,
+	})
+}
+
+// @Summary Update user preference settings
+// @ID update-user-preference-settings
+// @Security CoderSessionToken
+// @Accept json
+// @Produce json
+// @Tags Users
+// @Param user path string true "User ID, name, or me"
+// @Param request body codersdk.UpdateUserPreferenceSettingsRequest true "New preference settings"
+// @Success 200 {object} codersdk.UserPreferenceSettings
+// @Router /users/{user}/preferences [put]
+func (api *API) putUserPreferenceSettings(rw http.ResponseWriter, r *http.Request) {
+	var (
+		ctx  = r.Context()
+		user = httpmw.UserParam(r)
+	)
+
+	var params codersdk.UpdateUserPreferenceSettingsRequest
+	if !httpapi.Read(ctx, rw, r, &params) {
+		return
+	}
+
+	updatedTaskAlertDismissed, err := api.Database.UpdateUserTaskNotificationAlertDismissed(ctx, database.UpdateUserTaskNotificationAlertDismissedParams{
+		UserID:                         user.ID,
+		TaskNotificationAlertDismissed: params.TaskNotificationAlertDismissed,
+	})
+	if err != nil {
+		httpapi.Write(ctx, rw, http.StatusInternalServerError, codersdk.Response{
+			Message: "Internal error updating user task notification alert dismissed.",
+			Detail:  err.Error(),
+		})
+		return
+	}
+
+	httpapi.Write(ctx, rw, http.StatusOK, codersdk.UserPreferenceSettings{
+		TaskNotificationAlertDismissed: updatedTaskAlertDismissed,
+	})
+}
+
 func isValidFontName(font codersdk.TerminalFontName) bool {
 	return slices.Contains(codersdk.TerminalFontNames, font)
 }