web-ui: sanitize markdown HTML with DOMPurify to fix XSS

Both mdNode implementations (agent UI app.js, dashboard common.js)
assigned marked.parse() output straight to innerHTML with no
sanitizer. marked v5+ dropped its built-in sanitize option, and there
was no DOMPurify anywhere in frontend/, so markdown containing raw
HTML/script tags rendered live in the browser.

Both sinks receive untrusted input in practice: the agent UI's mdNode
renders recv tool_result bodies, assistant prose, and send/ask/answer
payloads sourced from peer agents and matrix-relayed messages (the
documented prompt-injection adversary); the dashboard's mdNode renders
agent-authored .md files served verbatim by GET /api/state-file
(the endpoint validates path, not content). Since the per-agent UI and
dashboard are same-origin behind the gateway with operator-authority
endpoints (approve/spawn/rebuild/destroy/answer-question), injected
script would run with the operator's session.

Fix: DOMPurify.sanitize() the marked.parse() output at both sinks
before assigning to innerHTML. Added dompurify as a dependency to
both the agent and dashboard npm workspaces, recomputed npmDepsHash
in nix/frontend.nix for the updated lockfile. Also corrected
docs/web-ui/shape.md, which claimed the markdown-rendering path was
XSS-safe by construction the same way the text-node-based linkify
path is — it isn't; it's safe because it's sanitized.

CSP hardening for the dashboard (no unsafe-inline) is a separate,
larger backend change (response headers in hive-c0re) and is left as
a fast-follow rather than folded into this fix.

frontend/packages/agent/package.json

@@ -9,7 +9,8 @@
   },
   "dependencies": {
     "@hive/shared": "*",
-    "marked": "18.0.4",
-    "chart.js": "4.5.1"
+    "chart.js": "4.5.1",
+    "dompurify": "^3.2.4",
+    "marked": "18.0.4"
   }
 }