feat(widevine): auto-seed CDM hint file for persistent contexts (Linux)

Sideloaded Widevine works on the first launch of a persistent context
instead of needing a manual two-launch hint-file workaround. The wrapper
writes Chromium's CDM hint file into the profile before launch when a
WidevineCdm directory is present next to the binary.

- New cloakbrowser/widevine.py and js/src/widevine.ts: resolve a sideloaded
  CDM (CLOAKBROWSER_WIDEVINE_CDM env var, else next to the binary) and seed
  the hint file. Linux only; no-op elsewhere. CLOAKBROWSER_WIDEVINE=0 disables.
- Never bundles/downloads/copies the CDM (proprietary); seeds only when the
  user-provided CDM is already present.
- Wired into launch_persistent_context[_async] and launchPersistentContext.
- README + js/README: Widevine / DRM section, env vars, FPJS tradeoff note.
- Tests: tests/test_widevine.py, js/tests/widevine.test.ts, persistent-context
  integration assertions.

js/src/playwright.ts

@@ -10,6 +10,7 @@ import { buildArgs } from "./args.js";
 import { ensureBinary } from "./download.js";
 import { resolveProxyConfig } from "./proxy.js";
 import { maybeResolveGeoip, resolveWebrtcArgs } from "./geoip.js";
+import { seedWidevineHint } from "./widevine.js";
 
 /** @internal Accept both timezone and timezoneId — either works, no warning. Exported for testing. */
 export function resolveTimezone<T extends { timezone?: string; timezoneId?: string }>(options: T): T {
@@ -227,6 +228,8 @@ export async function launchPersistentContext(
   }
   const args = buildArgs({ ...options, ...resolved, args: [...(resolvedArgs ?? []), ...proxyArgs] });
 
+  seedWidevineHint(options.userDataDir, binaryPath);
+
   // locale and timezone are set via binary flags (--lang, --fingerprint-timezone)
   // — NOT via Playwright context kwargs which use detectable CDP emulation.
   const context = await chromium.launchPersistentContext(options.userDataDir, {