Global configuration flag for media previews (#29582)

* Modify useMediaVisible to take a room.

* Add initial support for a account data level key.

* Update controls.

* Update settings

* Lint and fixes

* make some tests go happy

* lint

* i18n

* update preferences

* prettier

* Update settings tab.

* update screenshot

* Update docs

* Rewrite controller

* Rewrite tons of tests

* Rewrite RoomAvatar to be a functional component

This is so we can use hooks to determine the setting state.

* lint

* lint

* Tidy up comments

* Apply media visible hook to inline images.

* Move conditionals.

* copyright all the things

* Review changes

* Update html utils to properly discard media.

* Types fix

* Fixing tests that break settings getValue expectations

* Fix logic around media preview calculation

* Fix room header tests

* Fixup tests for timelinePanel

* Clear settings in matrixchat

* Update tests to use SettingsStore where possible.

* fix bug

* revert changes to client.ts

* copyright years

* Add header

* Add a test for MediaPreviewAccountSettingsTab

* Mark initMatrixClient as optional

* Improve on types

* Ensure we do not set the account data twice.

* lint

* Review changes

* Ensure we include the client on rendered messages.

* Fix test

* update labels

* clean designs

* update settings tab

* update snapshot

* copyright

* prevent mutation

src/@types/matrix-js-sdk.d.ts

@@ -1,5 +1,5 @@
 /*
-Copyright 2024 New Vector Ltd.
+Copyright 2024, 2025 New Vector Ltd.
 Copyright 2024 The Matrix.org Foundation C.I.C.
 
 SPDX-License-Identifier: AGPL-3.0-only OR GPL-3.0-only OR LicenseRef-Element-Commercial
@@ -14,6 +14,7 @@ import type { EncryptedFile } from "matrix-js-sdk/src/types";
 import type { EmptyObject } from "matrix-js-sdk/src/matrix";
 import type { DeviceClientInformation } from "../utils/device/types.ts";
 import type { UserWidget } from "../utils/WidgetUtils-types.ts";
+import { type MediaPreviewConfig } from "./media_preview.ts";
 
 // Extend Matrix JS SDK types via Typescript declaration merging to support unspecced event fields and types
 declare module "matrix-js-sdk/src/types" {
@@ -87,6 +88,8 @@ declare module "matrix-js-sdk/src/types" {
         "m.accepted_terms": {
             accepted: string[];
         };
+
+        "io.element.msc4278.media_preview_config": MediaPreviewConfig;
     }
 
     export interface AudioContent {

src/@types/media_preview.ts

@@ -0,0 +1,33 @@
+/*
+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.
+*/
+
+export enum MediaPreviewValue {
+    /**
+     * Media previews should be enabled.
+     */
+    On = "on",
+    /**
+     * Media previews should only be enabled for rooms with non-public join rules.
+     */
+    Private = "private",
+    /**
+     * Media previews should be disabled.
+     */
+    Off = "off",
+}
+
+export const MEDIA_PREVIEW_ACCOUNT_DATA_TYPE = "io.element.msc4278.media_preview_config";
+export interface MediaPreviewConfig extends Record<string, unknown> {
+    /**
+     * Media preview setting for thumbnails of media in rooms.
+     */
+    media_previews: MediaPreviewValue;
+    /**
+     * Media preview settings for avatars of rooms we have been invited to.
+     */
+    invite_avatars: MediaPreviewValue.On | MediaPreviewValue.Off;
+}