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/apidoc/docs.go

@@ -8387,6 +8387,84 @@ const docTemplate = `{
                 }
             }
         },
+        "/users/{user}/preferences": {
+            "get": {
+                "security": [
+                    {
+                        "CoderSessionToken": []
+                    }
+                ],
+                "produces": [
+                    "application/json"
+                ],
+                "tags": [
+                    "Users"
+                ],
+                "summary": "Get user preference settings",
+                "operationId": "get-user-preference-settings",
+                "parameters": [
+                    {
+                        "type": "string",
+                        "description": "User ID, name, or me",
+                        "name": "user",
+                        "in": "path",
+                        "required": true
+                    }
+                ],
+                "responses": {
+                    "200": {
+                        "description": "OK",
+                        "schema": {
+                            "$ref": "#/definitions/codersdk.UserPreferenceSettings"
+                        }
+                    }
+                }
+            },
+            "put": {
+                "security": [
+                    {
+                        "CoderSessionToken": []
+                    }
+                ],
+                "consumes": [
+                    "application/json"
+                ],
+                "produces": [
+                    "application/json"
+                ],
+                "tags": [
+                    "Users"
+                ],
+                "summary": "Update user preference settings",
+                "operationId": "update-user-preference-settings",
+                "parameters": [
+                    {
+                        "type": "string",
+                        "description": "User ID, name, or me",
+                        "name": "user",
+                        "in": "path",
+                        "required": true
+                    },
+                    {
+                        "description": "New preference settings",
+                        "name": "request",
+                        "in": "body",
+                        "required": true,
+                        "schema": {
+                            "$ref": "#/definitions/codersdk.UpdateUserPreferenceSettingsRequest"
+                        }
+                    }
+                ],
+                "responses": {
+                    "200": {
+                        "description": "OK",
+                        "schema": {
+                            "$ref": "#/definitions/codersdk.UserPreferenceSettings"
+                        }
+                    }
+                }
+            }
+        },
         "/users/{user}/profile": {
             "put": {
                 "security": [
@@ -19254,6 +19332,14 @@ const docTemplate = `{
                 }
             }
         },
+        "codersdk.UpdateUserPreferenceSettingsRequest": {
+            "type": "object",
+            "properties": {
+                "task_notification_alert_dismissed": {
+                    "type": "boolean"
+                }
+            }
+        },
         "codersdk.UpdateUserProfileRequest": {
             "type": "object",
             "required": [
@@ -19639,6 +19725,14 @@ const docTemplate = `{
                 }
             }
         },
+        "codersdk.UserPreferenceSettings": {
+            "type": "object",
+            "properties": {
+                "task_notification_alert_dismissed": {
+                    "type": "boolean"
+                }
+            }
+        },
         "codersdk.UserQuietHoursScheduleConfig": {
             "type": "object",
             "properties": {