Phase 4: embed UI into kamado-api binary

The production image is now a single Go binary that serves both the
JSON/WebSocket API under /api and the Svelte dashboard at /.

- New internal/webui package embeds a dist/ subdir via //go:embed.
  A placeholder index.html is committed so `go build` works on a
  fresh checkout; anything else in dist/ is regenerated per build
  and gitignored.

- httpapi.Server.Handler mounts the embed.FS at / with SPA-style
  fallback: unknown non-/api paths serve index.html so client-side
  routes survive a reload. /api/* is carved out explicitly so POSTs
  or typos never accidentally shadow API semantics with HTML.

- api/Dockerfile grows a node:22 builder stage that runs
  `npm ci && npm run build`, and the Go stage copies ui/dist/ into
  internal/webui/dist/ before `go build`. Build context moves to
  the repo root (docker-compose + `make api` both updated) so the
  Dockerfile can see both api/ and ui/.

With this in place, `make up` brings the whole stack online at
http://localhost:8080 — API under /api, dashboard at /. The Vite
dev server on :5173 with the /api proxy is still available via
`make ui-dev` for hot-reload development.

README.md

@@ -44,7 +44,7 @@ Three main components:
     - [ ] Phase 2b.5: ZMQ block notifier, SQLite persistence (deferred until s9pk repo exists — need real Go build env for new deps)
 - [x] ckpool patch 0001: expose `bestever` in runtime socket JSON so the UI can show "this round" and "all-time" best share side by side
 - [~] **Phase 3** — Svelte UI dashboard (skeleton: header, pool overview, miners table, blocks, best shares leaderboard; live WS updates)
-- [ ] **Phase 4** — Monorepo Docker build, full stack integration
+- [x] **Phase 4** — Monorepo Docker build: `kamado-api` embeds `ui/dist` via `//go:embed` and serves it at `/`. The api Dockerfile has a node stage that builds the UI before the Go stage embeds and builds the binary; docker-compose uses the repo root as build context so both `api/` and `ui/` are visible.
 - [ ] **Phase 5** — Testing (regtest, testnet4), polish
 
 ## Quick start (dev)