Replace markdown denylist sanitizer with vendored DOMPurify (mXSS hardening)

The previous _sanitizeHtml was a denylist over agent/transcript markdown
rendered via innerHTML; it missed style attributes and the svg/math mXSS
namespaces — e.g. <svg><style><img src=x onerror=alert(1)></style></svg>
re-serialized into a live <img onerror>.

Vendor DOMPurify 3.4.8 (allowlist) following the existing marked.min.js
vendor pattern (same-origin, CSP script-src 'self'; not in package.json so
no lockfile drift). New sanitize-html.js wires a hardened allowlist config
(FORBID style/svg/math/script/iframe/object/embed/form; no data attrs);
app.js _sanitizeHtml delegates to it with a fail-closed escape-all fallback.
index.html loads dompurify -> sanitize-html -> app.js (defer); build.mjs
minifies + content-hashes sanitize-html.js.

Test: test/markdown-sanitizer.test.ts (jsdom, real shipping artifacts) —
mXSS payloads neutralized + legit markdown preserved.

src/web/public/index.html

@@ -39,6 +39,9 @@
   <script defer src="vendor/xterm-addon-unicode11.min.js"></script>
   <script defer src="vendor/xterm-zerolag-input.js"></script>
   <script defer src="vendor/marked.min.js"></script>
+  <!-- DOMPurify (allowlist HTML sanitizer for rendered markdown).
+       Must load before sanitize-html.js (which wires it) and app.js (which calls it). -->
+  <script defer src="vendor/dompurify.min.js"></script>
   <!-- Synchronous mobile detection — runs before first paint to prevent panel flash -->
   <script>if(window.innerWidth<768||(('ontouchstart' in window||navigator.maxTouchPoints>0)&&window.innerWidth<1024))document.documentElement.classList.add('mobile-init');</script>
   <!-- Synchronous skin selection — runs before first paint to prevent theme flash -->
@@ -1907,6 +1910,8 @@
   <script defer src="notification-manager.js"></script>
   <script defer src="keyboard-accessory.js"></script>
   <script defer src="input-cjk.js"></script>
+  <!-- Hardened markdown HTML sanitizer (wires DOMPurify). Must precede app.js. -->
+  <script defer src="sanitize-html.js"></script>
   <script defer src="app.js"></script>
   <script defer src="terminal-ui.js"></script>
   <script defer src="respawn-ui.js"></script>