Phase 2 of #273. Adds `packages.${system}.frontend` to the flake —
a `buildNpmPackage` derivation that consumes the lockfile committed
in the previous step and produces two static dist trees under $out:
$out/dashboard/ the hive-c0re dashboard SPA assets
(index.html, app.js, dashboard.css, favicon.svg)
$out/agent/ the per-agent default UI assets
(index.html, app.js, stats.html, stats.js,
agent.css, screen.html)
The dashboard favicon lives outside the frontend src tree
(branding/hyperhive.svg at the repo root). It's passed in as a
callPackage argument so the hermetic build can grab it.
`npmDepsHash` is set to `lib.fakeHash` — the build will fail on
first attempt with the actual sha256 printed; copy that in. Use
`nix run nixpkgs#prefetch-npm-deps -- frontend/package-lock.json`
to recompute locally without a build round-trip (works from
operator's host; iris's container can't recompute it without
prefetch-npm-deps in PATH).
The Rust crates and NixOS modules continue to use the legacy
include_str! routes; cutover happens in Phase 4.
Refs #273.
