feat: auto-install Playwright browsers on launch failure

When the browser binary is missing (fresh install, first run), detect
the error and run `playwright install <channel>` automatically instead
of erroring out. On Linux with root, also attempts `install-deps`.

- New src/browserInstaller.ts module with install deduping and caching
- Wired into isolated and persistent context launch paths
- Wired into video-recording launch path in context.ts
- Clear error if playwright npm package itself isn't resolvable

src/browserInstaller.ts

@@ -0,0 +1,118 @@
+/**
+ * Copyright (c) Microsoft Corporation.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { fork } from 'node:child_process';
+import path from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+import { testDebug } from './log.js';
+
+import type { FullConfig } from './config.js';
+
+// Cache installs per-process so repeated launches don't re-run the installer.
+const installedBrowsers = new Set<string>();
+const inflightInstalls = new Map<string, Promise<void>>();
+
+function browserTarget(browserConfig: FullConfig['browser']): string {
+  return browserConfig.launchOptions?.channel
+    ?? browserConfig.browserName
+    ?? 'chrome';
+}
+
+/**
+ * Runs `playwright install <target>` in a forked child process.
+ * Safe to call concurrently — in-flight installs are deduplicated per target.
+ */
+export async function installBrowser(browserConfig: FullConfig['browser']): Promise<void> {
+  const target = browserTarget(browserConfig);
+
+  if (installedBrowsers.has(target))
+    return;
+
+  const existing = inflightInstalls.get(target);
+  if (existing)
+    return existing;
+
+  const promise = runInstall(target).then(() => {
+    installedBrowsers.add(target);
+  }).finally(() => {
+    inflightInstalls.delete(target);
+  });
+
+  inflightInstalls.set(target, promise);
+  return promise;
+}
+
+async function runInstall(target: string): Promise<void> {
+  testDebug(`auto-installing browser: ${target}`);
+  const cliPath = resolvePlaywrightCli();
+
+  await runPlaywrightCli(cliPath, ['install', target]);
+
+  // Best-effort system-deps install. Only runs when we're already root,
+  // otherwise skipped silently — users will see Playwright's own missing-lib
+  // error on the next launch, which tells them exactly what to apt install.
+  if (process.getuid && process.getuid() === 0) {
+    try {
+      await runPlaywrightCli(cliPath, ['install-deps', target]);
+    } catch (e) {
+      testDebug(`install-deps failed (non-fatal): ${e}`);
+    }
+  }
+}
+
+function resolvePlaywrightCli(): string {
+  try {
+    const cliUrl = import.meta.resolve('playwright/package.json');
+    return path.join(fileURLToPath(cliUrl), '..', 'cli.js');
+  } catch (e) {
+    throw new Error(
+        'Playwright package not found. Install it with: npm install playwright\n' +
+      `Original error: ${e instanceof Error ? e.message : String(e)}`
+    );
+  }
+}
+
+function runPlaywrightCli(cliPath: string, args: string[]): Promise<void> {
+  const child = fork(cliPath, args, { stdio: 'pipe' });
+  const output: string[] = [];
+  child.stdout?.on('data', data => output.push(data.toString()));
+  child.stderr?.on('data', data => output.push(data.toString()));
+
+  return new Promise<void>((resolve, reject) => {
+    child.on('close', code => {
+      if (code === 0) {
+        resolve();
+        return;
+      }
+      reject(new Error(`playwright ${args.join(' ')} failed (exit ${code}):\n${output.join('')}`));
+    });
+    child.on('error', reject);
+  });
+}
+
+/**
+ * Returns true if the given error indicates the browser executable is missing
+ * and needs to be downloaded via `playwright install`.
+ */
+export function isMissingBrowserError(error: unknown): boolean {
+  if (!(error instanceof Error))
+    return false;
+  const msg = error.message;
+  return msg.includes("Executable doesn't exist")
+    || msg.includes('please run the following command')
+    || msg.includes('npx playwright install');
+}