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/package-lock.json

@@ -476,6 +476,13 @@
       "integrity": "sha512-M5UknZPHRu3DEDWoipU6sE8PdkZ6Z/S+v4dD+Ke8IaNlpdSQah50lz1KtcFBa2vsdOnwbbnxJwVM4wty6udA5w==",
       "license": "MIT"
     },
+    "node_modules/@types/trusted-types": {
+      "version": "2.0.7",
+      "resolved": "https://registry.npmjs.org/@types/trusted-types/-/trusted-types-2.0.7.tgz",
+      "integrity": "sha512-ScaPdn1dQczgbl0QFTeTOmVHFULt394XJgOQNoyVhZ6r2vLnMLJfBPd53SB52T/3G36VI1/g2MZaX0cwDuXsfw==",
+      "license": "MIT",
+      "optional": true
+    },
     "node_modules/chart.js": {
       "version": "4.5.1",
       "resolved": "https://registry.npmjs.org/chart.js/-/chart.js-4.5.1.tgz",
@@ -488,6 +495,15 @@
         "pnpm": ">=8"
       }
     },
+    "node_modules/dompurify": {
+      "version": "3.2.4",
+      "resolved": "https://registry.npmjs.org/dompurify/-/dompurify-3.2.4.tgz",
+      "integrity": "sha512-ysFSFEDVduQpyhzAob/kkuJjf5zWkZD8/A9ywSp1byueyuCfHamrCBa14/Oc2iiB0e51B+NpxSl5gmzn+Ms/mg==",
+      "license": "(MPL-2.0 OR Apache-2.0)",
+      "optionalDependencies": {
+        "@types/trusted-types": "^2.0.7"
+      }
+    },
     "node_modules/esbuild": {
       "version": "0.28.0",
       "resolved": "https://registry.npmjs.org/esbuild/-/esbuild-0.28.0.tgz",
@@ -548,6 +564,7 @@
       "dependencies": {
         "@hive/shared": "*",
         "chart.js": "4.5.1",
+        "dompurify": "^3.2.4",
         "marked": "18.0.4"
       }
     },
@@ -556,6 +573,7 @@
       "version": "0.0.0",
       "dependencies": {
         "@hive/shared": "*",
+        "dompurify": "^3.2.4",
         "marked": "18.0.4"
       }
     },