fix: 🐛 use pagehide for lifecycle cleanup

[aermin]

Summary

Fixes #56 by replacing lifecycle cleanup that relied on the deprecated unload event with pagehide.

Why

Registering unload handlers can trigger Chrome permissions policy warnings. Unlike unload, pagehide also reports whether a page is entering the back-forward cache through event.persisted.

Changes

• Stop registering unload handlers for heartbeat and lock cleanup.
• Clean up heartbeat timers and this client's locks on non-BFCache pagehide.
• Preserve the current lifecycle state when pagehide.persisted === true.
• Add Jest regression coverage in test/bfcache.test.ts for both lifecycle branches.

The tests exercise this polyfill's lifecycle handling; existing cross-context heartbeat behavior is unchanged.

Validation

• npm test -- --runInBand (13 suites, 68 tests)
• npm run lint
• npm run build