refactor: merge storybook build and test into single job

Eliminates duplicate Storybook build step, saving ~1m21s in CI.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: Simon Bigelmayr

.github/workflows/validate.yml

@@ -51,17 +51,6 @@ jobs:
       - run: yarn run build
 
   storybook:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v5
-      - uses: actions/setup-node@v6
-        with:
-          node-version: 22
-      - run: corepack enable
-      - run: yarn install
-      - run: yarn run storybook:build
-
-  test-storybook:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v5