fix(web): cross-platform sync-assets + surface build errors on failure

Three Windows-only bugs in the web-dashboard build path. Each is small,
scoped, and verified end-to-end on Windows 11 — including under a stock
cmd.exe / PowerShell console with its default cp1252 encoding.

1. `sync-assets` shells out to Unix-only commands

   web/package.json hard-codes `rm -rf … && cp -r …`. Neither exists on
   Windows cmd.exe. `hermes_cli/main.py::_build_web_ui` runs npm via
   subprocess (which on Windows defaults to cmd.exe), so the prebuild
   hook crashed before Vite ever ran and the dashboard never built.

   Fix: web/scripts/sync-assets.mjs — ~20 lines of Node using fs.rmSync
   + fs.cpSync (stdlib, Node >= 16.7). No new deps, identical behavior
   on POSIX and Windows.

2. Build failures were silent

   _build_web_ui ran both subprocess calls with capture_output=True and
   never relayed the captured buffers on failure. Users saw 'Web UI
   build failed' and nothing else — no stdout, no stderr, no hint that
   the real problem was 'rm is not recognized'.

   Fix: inner _relay() helper that decodes and prints stdout + stderr
   (utf-8, errors='replace') whenever a step returns non-zero. Replaces
   the existing stderr_tail-only relay on the build path; success path
   is unchanged. (stderr_tail is preserved for the stale-dist fallback
   branch added by #23817.)

Salvaged from #13368 by @johnisag onto current main. Conflict
resolution preserves main's improvements:
- _run_npm_install_deterministic() (replaces bare subprocess.run for
  npm install)
- npm-build retry-after-sleep for Windows boot-time races (#23817)
- stale-dist fallback for non-interactive callers (#23817)

Closes #25073, #13368.

web/package.json

@@ -4,7 +4,7 @@
   "version": "0.0.0",
   "type": "module",
   "scripts": {
-    "sync-assets": "node -e \"const fs=require('fs');fs.rmSync('public/fonts',{recursive:true,force:true});fs.rmSync('public/ds-assets',{recursive:true,force:true});fs.cpSync('node_modules/@nous-research/ui/dist/fonts','public/fonts',{recursive:true});fs.cpSync('node_modules/@nous-research/ui/dist/assets','public/ds-assets',{recursive:true});\"",
+    "sync-assets": "node scripts/sync-assets.mjs",
     "predev": "npm run sync-assets",
     "prebuild": "npm run sync-assets",
     "dev": "vite",