feat: harden service worker cache invalidation and update prompts (#680)

[knoxiboy]

Description

Resolves #680

This PR hardens Service Worker registration update handling. It wires listeners for updatefound and statechange events, detecting when a new service worker transitions to the installed (waiting) state. It then displays a floating toast/prompt alerting the user that a new version of ReGenX is available and offers a "Reload" button to trigger activation.

What changed

• Added showUpdatePrompt and wired check for reg.waiting on load inside src/app.js.
• Implemented updatefound update triggers to detect when a new worker becomes available.
• Removed duplicate registration of /service-worker.js at the end of src/app.js.

Why

When the Service Worker is updated with new modules, files in the cache might mismatch with active client modules, leading to blank screens or load crashes if not immediately activated and reloaded.

How to test

1. Modify service-worker.js or change CACHE_VERSION value.
2. Load the application in browser.
3. Verify that the "Update Available!" toast with a reload button is displayed once the new service worker installation completes.

Checklist

• Tested changes locally
• No lint/syntax errors introduced
• Matches all issue criteria

Files Affected

• src/app.js

[knoxiboy]

@Shruti070107 Please review this pr and merge it

[knoxiboy]

@Shruti070107 Please review this pr and merge it