swarm-ui: real hive-roster overview page

Fixes hyperhive#3223.

swarm-controller: GET /api/hives (utoipa-annotated same as /health),
serving the swarm's hive directory (name + domain) loaded once at
startup from a new SWARM_CONTROLLER_HIVES env var. The controller's
NixOS module sets it from services.hyperhive.swarm.hives, JSON-encoded
the same way hive-c0re already builds HYPERHIVE_PEERS for its own peer
list (environment.nix) — the full directory here rather than
peers-minus-self, since a swarm-level daemon has no 'self' hive to
exclude. Unset/malformed both fall back to an empty list with a
warning rather than failing startup, so /health stays answerable even
if this one env var is wrong.

swarm-ui: App.tsx's Home route fetches /api/hives and renders it
through the already-merged <Table>/<StatusChip>/<Panel> primitives —
name, domain (linking out to that hive's own gateway-routed
dashboard), and a static "configured" status chip until a real
online/stale/offline rollup exists server-side. Also gave swarm-ui a
base <a> color (theme's --blue) — base.css covers body/typography but
not links, and this is genuinely page-level rather than any one
component's concern.

Verified end to end, not just source-reading: ran the real
swarm-controller binary with SWARM_CONTROLLER_HIVES set, curled
/api/hives + /health over its actual unix socket; separately served
the real swarm-ui dist against a mock /api/hives and screenshotted the
rendered table. Also re-verified the nginx wiring evaluates (same
throwaway nixosSystem eval technique as #3212) — SWARM_CONTROLLER_HIVES
resolves to the expected JSON shape.

cargo test/clippy -p swarm-controller clean (2 tests, including a new
load_hives one covering missing/malformed/valid env var states). npm
run build + typecheck clean.

Cargo.lock

@@ -4427,6 +4427,8 @@ version = "0.1.0"
 dependencies = [
  "anyhow",
  "axum",
+ "serde",
+ "serde_json",
  "tokio",
  "tracing",
  "tracing-subscriber",