Update the mobile_guide page to the new design and link out to Element X by default. (#30172)

* Reapply "Update the mobile_guide page to the new design. (#30006)" (#30104)

This reverts commit c51823db5e.

* Use Element X as the default mobile_guide_app_variant when omitted.

* Fix a build error on Windows.

Additionally revert "Remove unnecessary <%= require %> usages" and let webpack handle all of the assets (without a manual copy rule).

* Exclude mobile_guide from coverage gate

It has playwright tests

* Revert the re-introduction of <%= require %>

* Fix snapshot tests on mobile_guide.

---------

Co-authored-by: Michael Telatynski <7t3chguy@gmail.com>

playwright/e2e/mobile-guide/mobile-guide.spec.ts

@@ -0,0 +1,36 @@
+/*
+Copyright 2025 New Vector Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR GPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE files in the repository root for full details.
+*/
+
+import { test, expect } from "../../element-web-test";
+import { MobileAppVariant } from "../../../src/vector/mobile_guide/mobile-apps";
+
+const variants = [MobileAppVariant.Classic, MobileAppVariant.X, MobileAppVariant.Pro];
+
+test.describe("Mobile Guide Screenshots", { tag: "@screenshot" }, () => {
+    for (const variant of variants) {
+        test.describe(`for variant ${variant}`, () => {
+            test.use({
+                config: {
+                    default_server_config: {
+                        "m.homeserver": {
+                            base_url: "https://matrix.server.invalid",
+                            server_name: "server.invalid",
+                        },
+                    },
+                    mobile_guide_app_variant: variant,
+                },
+                viewport: { width: 390, height: 844 }, // iPhone 16e
+            });
+
+            test("should match the mobile_guide screenshot", async ({ page, axe }) => {
+                await page.goto("/mobile_guide/");
+                await expect(page).toMatchScreenshot(`mobile-guide-${variant}.png`);
+                await expect(axe).toHaveNoViolations();
+            });
+        });
+    }
+});