fix(ci): add missing frontend dependency installation in E2E workflow

The E2E workflow was failing during "Build frontend" because npm ci
was only run at root level. The frontend directory has its own
package.json with React, Tailwind, and other dependencies that were
never installed.

Add "Install frontend dependencies" step before build
Update Node.js version from 18 to 20 (required by markdownlint-cli2)
Fixes failing E2E tests in PR #550

.github/workflows/e2e-tests.yml

@@ -58,7 +58,7 @@ on:
           - all
 
 env:
-  NODE_VERSION: '18'
+  NODE_VERSION: '20'
   GO_VERSION: '1.21'
   REGISTRY: ghcr.io
   IMAGE_NAME: ${{ github.repository_owner }}/charon
@@ -98,6 +98,10 @@ jobs:
       - name: Install dependencies
         run: npm ci
 
+      - name: Install frontend dependencies
+        run: npm ci
+        working-directory: frontend
+
       - name: Build frontend
         run: npm run build
         working-directory: frontend