Prevent PWA version skew from disabling the UI

[aliorhun]

Summary

Fixes the case where the page renders but all controls appear dead after a deploy.

Root cause

During a PWA update, an older cached HTML shell can briefly run with a newer JavaScript bundle. The newer bundle treated newly-added controls such as examples-btn / backup controls as mandatory and called .addEventListener() or updated text on a null element. That runtime exception stops the bundle before the rest of the UI finishes wiring.

Changes

• make newly-added Examples and Backup controls null-safe
• keep the rest of the app usable if an older cached HTML shell is paired with newer JS
• stop the service worker from using skipWaiting() / clients.claim(), so it no longer takes over an already-open page mid-session
• fetch mutable core assets network-first with cache: 'no-store'
• keep offline fallback and precaching
• extend verification to catch unsafe optional UI bindings and eager service-worker takeover

This is a compatibility hotfix for GitHub Pages/PWA deploy transitions.