Fix/proxy scores sort mutation

[MayurK-cmd]

What does this PR do?

Fixes #47
Fixes an impure render in webapp/src/pages/ProxyScores.tsx where the per-host
proxy score array was sorted in-place (data.scores[host].sort(...)) during
render, mutating state derived from the API response. Under <StrictMode> the
render runs twice in dev, amplifying the side effect.

The sort is now applied to a copy of the array:

const scores = [...data.scores[host]].sort((a, b) => b.score - a.score);

No behavior change — the displayed order is identical.

How to test

1. Start the server with PROXY_URLS configured and scrape a few URLs so
   scores are populated.
2. Open the web dashboard → Proxy Scores and confirm scores still render
   best-first per host (unchanged ordering).
3. In dev, confirm no render-time mutation warnings appear under StrictMode.

Checklist

• Tests pass (cd server && go test ./...) — no Go code changed
• No breaking changes to existing API endpoints
• Documentation updated — N/A