feat: refactor countdown update in AFKOverlay

[odaysec]

Refactor updateCountdown method to use textContent for countdown updates.

fix unsafe HTML construction, avoid writing untrusted data into innerHTML. Instead, build the DOM structure using document.createElement, and insert dynamic pieces via textContent/innerText or by explicitly setting properties on elements. This prevents the dynamic value from being interpreted as HTML and eliminates the XSS sink.

For this specific case, the overlay content is simple and mostly static. The only dynamic part is the numeric countdown. The safest and least intrusive fix is:

1. Ensure the base DOM structure, including the <span id="afkCountDownNumber">, is created without dynamic content (this is already done in createContentElement).
2. Change updateCountdown so it does not rebuild the full HTML string with innerHTML. Instead, locate the existing <span id="afkCountDownNumber"> within this.textElement and update its textContent with the current countdown value.
3. Keep the method signature and external behavior (the text the user sees) functionally equivalent, just implemented via DOM manipulation instead of string concatenation.

Concretely, in Frontend/ui-library/src/Overlay/AFKOverlay.ts, replace line 47’s innerHTML assignment with logic like:

• Find the span: const span = this.textElement.querySelector('#afkCountDownNumber') as HTMLSpanElement | null;
• If it exists, set span.textContent = String(countdown);
• Optionally, if the span is missing (e.g. defensive), fall back to updating this.textElement.textContent or do nothing; here, a simple defensive check is enough.

No changes are needed in Frontend/ui-library/src/Application/Application.ts, because after the AFKOverlay is made safe, passing countDown into updateCountdown no longer reaches a dangerous sink.

Relevant components:

• Signalling server
• Common library
• Frontend library
• Frontend UI library
• Matchmaker
• Platform scripts
• SFU

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[lukehb]

I have highlighted an issue with the PR that would be need to be addressed before we could accept it.

Additionally, our linter has highlighted some issues that would also need to be addressed.