fix(vidstack): handle fullscreen dispose rejection

[mihar-22]

Summary

• Make the fullscreen dispose handler synchronous.
• Explicitly sink exit() rejections with void this.exit().catch(noop) during disposal.

Fixes #1789

Validation

• pnpm --filter vidstack typecheck passed.
• Initial validation attempt failed because the isolated worktree had no node_modules and tsgo was missing; installed dependencies with pnpm install --frozen-lockfile and reran successfully.