Add frontend build job to PR workflow (#1099)

## Summary

Adds a `frontend-build` job to the PR Build and Test workflow
(`.github/workflows/PR-Build-And-Test.yml`) so that npm package and Vite
build issues are caught on every PR before they reach the full
build/deploy pipeline.

## Changes

- New `frontend-build` job that:
  1. Checks out the repo
2. Sets up Node.js 26 (matching the Dockerfile's
`node:26-bookworm-slim`)
  3. Caches npm dependencies via `package-lock.json`
  4. Runs `npm ci` in `EssentialCSharp.Web/`
  5. Runs `npm run build` (Vite build) in `EssentialCSharp.Web/`

This mirrors the `frontend-build` stage in the Dockerfile exactly,
ensuring PRs are validated against the same build path used in
production.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: BenjaminMichaelis <22186029+BenjaminMichaelis@users.noreply.github.com>

Author: Copilot

.github/workflows/PR-Build-And-Test.yml

@@ -7,6 +7,26 @@ on:
   workflow_dispatch:
 
 jobs:
+  frontend-build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6
+
+      - name: Set up Node.js
+        uses: actions/setup-node@v6
+        with:
+          node-version: '26'
+          cache: npm
+          cache-dependency-path: EssentialCSharp.Web/package-lock.json
+
+      - name: Install npm dependencies
+        working-directory: EssentialCSharp.Web
+        run: npm ci
+
+      - name: Build frontend
+        working-directory: EssentialCSharp.Web
+        run: npm run build
+
   build-and-test:
     runs-on: ubuntu-latest
     steps: