[MM-T5139] Add Standard message priority and system setting test (#29526)

* add Standard message priority and system setting test

* Refactor message priority config and test.

* Refactor to use new priority dialog locators and checks, ensuring default standard option is correctly verified.

* Fix whitespace and formatting in message priority components.

---------

Co-authored-by: Fume <contact@fumedev.com>
Co-authored-by: Mattermost Build <build@mattermost.com>

e2e-tests/playwright/tests/functional/channels/message_priority/standard_priority.spec.ts

@@ -0,0 +1,51 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+import {expect, test} from '@e2e-support/test_fixture';
+import MessagePriority from '@e2e-support/ui/components/channels/message_priority';
+
+test('MM-T5139: Message Priority - Standard message priority and system setting', async ({pw, pages}) => {
+    // # Setup test environment
+    const {user} = await pw.initSetup();
+
+    // # Log in as a user in new browser context
+    const {page} = await pw.testBrowser.login(user);
+
+    // # Visit default channel page
+    const channelPage = new pages.ChannelsPage(page);
+    await channelPage.goto();
+    await channelPage.toBeVisible();
+
+    const messagePriority = new MessagePriority(page.locator('body'));
+
+    // Open menu
+    await channelPage.centerView.postCreate.openPriorityMenu();
+
+    // Use messagePriority for dialog interactions
+    await messagePriority.verifyPriorityDialog();
+    await messagePriority.verifyStandardOptionSelected();
+
+    // # Close menu and post message
+    await channelPage.centerView.postCreate.priorityButton.click();
+    const testMessage = 'This is just a test message';
+    await channelPage.postMessage(testMessage);
+
+    // # Verify message posts without priority label
+    const lastPost = await channelPage.centerView.getLastPost();
+    await lastPost.toBeVisible();
+    await lastPost.toContainText(testMessage);
+    await expect(lastPost.container.locator('.post-priority')).not.toBeVisible();
+
+    // # Open post in RHS and verify
+    await lastPost.container.click();
+    await channelPage.sidebarRight.toBeVisible();
+
+    // # Get RHS post and verify content
+    const rhsPost = await channelPage.sidebarRight.getLastPost();
+    await rhsPost.toBeVisible();
+    await rhsPost.toContainText(testMessage);
+    await expect(rhsPost.container.locator('.post-priority')).not.toBeVisible();
+
+    // # Verify RHS formatting bar doesn't have priority button
+    await expect(channelPage.sidebarRight.postCreate.priorityButton).not.toBeVisible();
+});