/share?msg=foo endpoint using forward message dialog (#29874)

* basic implementation of an /share?msg=foo endpoint

* SharePayload

* add sharing html & md while we're at it

* remove whitespace from imports to appease linter

* lint

* Add unit test

* More tests

* Test for showScreen

* Use one of the typed strings

* Test nasty tags stripped out

* Add playwright test

* Fix flake

by not relying on the name being synced as soon as we load

---------

Co-authored-by: David Baker <dbkr@users.noreply.github.com>

playwright/e2e/share-dialog/share-by-url.spec.ts

@@ -0,0 +1,34 @@
+/*
+ * 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";
+
+test.describe("share from URL", () => {
+    test.use({
+        displayName: "Alice",
+        room: async ({ app }, use) => {
+            const roomId = await app.client.createRoom({ name: "A test room" });
+            await use({ roomId });
+        },
+    });
+
+    test("should share message when users navigates to share URL", async ({ page, user, room, app }) => {
+        await page.goto("/#/share?msg=Hello+world");
+        // The forward message dialog doesn't update as new infomation arrives via sync, which means sometimes
+        // this is just says, "Empty room". For the same reason, we can't reliably write a test for loading the
+        // app straight away with a /#/share url as the room doesn't appear until the client syncs.]
+        // Ideally we should fix the forward dialog to update and eliminate races, until then, there is only one
+        // room so we click the first button.
+        await page.getByRole("listitem" /*, { name: "A test room" }*/).getByRole("button", { name: "Send" }).click();
+        await page.keyboard.press("Escape");
+        await app.viewRoomByName("A test room");
+        const lastMessage = page.locator(".mx_RoomView_MessageList .mx_EventTile_last");
+        await expect(lastMessage).toBeVisible();
+        const lastMessageText = await lastMessage.locator(".mx_EventTile_body").innerText();
+        await expect(lastMessageText).toBe("Hello world");
+    });
+});