feat: add an idempotent menubar teardown API

[Parthu-M]

Summary

• add idempotent destroy() and isDestroyed() APIs
• remove only the app and tray listeners registered by the Menubar instance
• clear pending timers, destroy the BrowserWindow, and release an owned Tray
• preserve a caller-supplied Tray while detaching Menubar's listeners
• document the lifecycle APIs and add regression coverage

Why

Applications that switch between tray and regular-window modes currently have no supported way to tear down a Menubar instance. This provides explicit, repeatable cleanup without using removeAllListeners(), which could remove listeners owned by the consuming application.

Fixes #281.

Validation

• corepack yarn test src/Menubar.spec.ts --runInBand --modulePathIgnorePatterns=lib (7 tests passed)
• npx --yes --package=typescript@5.9.2 tsc --noEmit --skipLibCheck
• corepack yarn biome check src/Menubar.ts src/Menubar.spec.ts src/__mocks__/electron.ts --formatter-enabled=false --max-diagnostics=100 (no errors)