Add renderer policy and update notifications

[SergeiNikolenko]

Summary

• add a shared renderer policy resolver for the app and Quick Look paths
• show in-app update availability from GitHub release metadata
• keep Quick Look fallback behavior intact while routing renderer preferences consistently
• bump version to 0.10.21

Testing

• npm run check:release
• npm run check:js

Notes

• PR Unify renderer policy resolution #31 and PR Show update notifications in the menu #32 were stacked and merged into this branch; this PR is the final integration target for main.