Security: add SRI integrity attributes to CDN-loaded highlight.js, marked.js, and theme CSS

[timon0305]

Problem

Three CDN-loaded assets in static/index.html ship without integrity (SRI) attributes:

<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.9.0/styles/vs2015.min.css" id="hljs-theme">
<script src="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.9.0/highlight.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/marked/12.0.1/marked.min.js"></script>

If cdnjs is compromised (or any CDN edge serving us the file is), the browser will execute whatever JavaScript is delivered — Marked.js parses raw markdown into HTML and highlight.js runs on every code block, so a swapped payload runs in our origin with full DOM access. SRI (integrity="sha384-..." + crossorigin="anonymous") tells the browser to refuse anything whose hash doesn't match a known-good value.

Wrinkle: runtime theme swap

static/js/app.js:128-131 (and duplicate at 839-843) toggles the hljs-theme <link> between two stylesheets at runtime:

hljsLink.href = theme === 'dark'
    ? 'https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.9.0/styles/vs2015.min.css'
    : 'https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.9.0/styles/github.min.css';

If we set integrity="sha384-<vs2015 hash>" on the static tag and the user toggles to light theme at runtime, the swap rewrites href but leaves the wrong-hash integrity in place. The browser then refuses the new stylesheet → broken UI.

The fix has to update both href and integrity together. A small const map of { href, integrity } keyed by theme name is the right shape.

Suggested fix

1. Add integrity="sha384-<hash>" crossorigin="anonymous" to all three static CDN tags in index.html. Hashes verified against the cdnjs SRI API (https://api.cdnjs.com/libraries/<name>/<version>?fields=sri).
2. Replace the inline ternary in app.js with a const map containing both { href, integrity } pairs; the theme swap reads from the map and assigns both attributes together.
3. Same fix applied to both call sites (app.js:128 and app.js:839).
4. (Optional follow-up, separate issue) github.min.css becomes a runtime-needed asset with its own SRI; consider preloading it via <link rel="preload" as="style"> so first toggle is instant.

Severity

Medium — Listed as Medium / 1pt in Will's eval week-1 plan for claude-code-chat-browser. Defence in depth: the dashboard already loads only over 127.0.0.1 and the CDN is reputable, but pinning hashes neutralises a whole class of supply-chain risk for ~6 lines of HTML / 10 lines of JS.