build(ui): fail fast when Admin UI build prerequisites are missing

Make `make build-ui` exit non-zero with an actionable message when npm is
absent (with a SKIP_UI_BUILD=1 escape hatch for headless deployments), so
`make install-dev` surfaces missing prerequisites at install time instead of
leaving /admin to fail at runtime with "No bundle-*.js found".

Provision Node via the official devcontainers feature so Codespaces /
devcontainer setup still completes end-to-end under the new fail-fast
behavior, and document the prerequisite + bypass in the building guide.

Signed-off-by: Jonathan Springer <jps@s390x.com>

Author: jonpspri

.devcontainer/devcontainer.json

@@ -4,7 +4,11 @@
     "dockerfile": "Dockerfile",
     "context": ".."
   },
-  "features": {},
+  "features": {
+    "ghcr.io/devcontainers/features/node:1": {
+      "version": "lts"
+    }
+  },
   "postCreateCommand": ".devcontainer/postCreateCommand.sh",
   "customizations": {
     "vscode": {

docs/docs/development/building.md

@@ -103,16 +103,24 @@ npm install        # install frontend dev dependencies
 
 ### Building the Admin UI Bundle
 
-The Admin UI JavaScript is bundled with Vite. The bundle is not committed to the repository — it is built locally during `make install-dev` (which calls `make build-ui`). Rebuild after any changes to files under `mcpgateway/admin_ui/`.
+The Admin UI JavaScript is bundled with Vite. The bundle is **not committed** to the repository — it must be built locally. `make install-dev` invokes `make build-ui` automatically, and the step is required: without a bundle, `/admin` will fail to load at runtime with `No bundle-*.js found`. Rebuild after any changes to files under `mcpgateway/admin_ui/`.
+
+**Node.js is a prerequisite.** If `npm` is missing, `make build-ui` (and therefore `make install-dev`) fails fast rather than leaving a broken install. Install Node.js from <https://nodejs.org> before running setup.
 
 ```bash
 make build-ui          # build mcpgateway/static/bundle-<hash>.js (requires npm)
 ```
 
-Or manually:
+If you genuinely do not need the Admin UI (for example, a headless API-only deployment where `MCPGATEWAY_UI_ENABLED=false`), bypass the step with:
+
+```bash
+SKIP_UI_BUILD=1 make install-dev
+```
+
+Or build manually:
 
 ```bash
-npm ci                 # install dependencies from lockfile (if not already done)
+npm ci                 # install dependencies from lockfile (falls back to `npm install` if lockfile absent)
 npm run vite:build     # produce mcpgateway/static/bundle-<hash>.js
 ```