perf(embed): improve embed performance (#14613)

* wip

* wip

* wip

* refactor

* refactor

---------

Co-authored-by: かっこかり <67428053+kakkokari-gtyih@users.noreply.github.com>

packages/frontend-embed/src/server-context.ts

@@ -0,0 +1,21 @@
+/*
+ * SPDX-FileCopyrightText: syuilo and misskey-project
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+import * as Misskey from 'misskey-js';
+
+const providedContextEl = document.getElementById('misskey_embedCtx');
+
+export type ServerContext = {
+	clip?: Misskey.entities.Clip;
+	note?: Misskey.entities.Note;
+	user?: Misskey.entities.UserLite;
+} | null;
+
+// NOTE: devモードのときしか embedCtx が null になることは無い
+export const serverContext: ServerContext = (providedContextEl && providedContextEl.textContent) ? JSON.parse(providedContextEl.textContent) : null;
+
+export function assertServerContext<K extends keyof NonNullable<ServerContext>>(ctx: ServerContext, entity: K): ctx is Required<Pick<NonNullable<ServerContext>, K>> {
+	if (ctx == null) return false;
+	return entity in ctx;
+}