chore: import upstream snapshot with attribution

examples/slack/app/render/browser.ts

@@ -0,0 +1,36 @@
+/**
+ * A single shared headless Chromium for local chart/diagram rendering.
+ *
+ * The rendering happens entirely in-process (our own browser) — only the
+ * charting *library* is fetched from a CDN; the user's data never leaves the
+ * host. Reuse one browser across renders so we don't pay a launch per call;
+ * `closeBrowser()` is wired into the bridge's shutdown.
+ *
+ * Requires a Chromium binary: `npx playwright install chromium`.
+ */
+import { chromium, type Browser } from "playwright";
+
+let browserPromise: Promise<Browser> | undefined;
+let closing = false;
+
+export function getBrowser(): Promise<Browser> {
+  if (closing) {
+    return Promise.reject(new Error("renderer is shutting down"));
+  }
+  if (!browserPromise) {
+    browserPromise = chromium.launch({ args: ["--no-sandbox"] });
+  }
+  return browserPromise;
+}
+
+export async function closeBrowser(): Promise<void> {
+  if (!browserPromise) return;
+  // Claim shutdown and detach the promise BEFORE awaiting, so a concurrent
+  // getBrowser() rejects cleanly instead of handing back a browser we're
+  // about to close out from under it.
+  closing = true;
+  const pending = browserPromise;
+  browserPromise = undefined;
+  const b = await pending.catch(() => undefined);
+  await b?.close().catch(() => {});
+}