fix(ci): build datum-ui before storybook in CI workflow

CI was missing the datum-ui build step, causing CSS dist files and
grid/nprogress exports to be unavailable. Added explicit build step
in workflow and prebuild script in storybook package.json as fallback.

Author: yahyafakhroji

.github/workflows/storybook.yml

@@ -19,4 +19,5 @@ jobs:
           node-version: 22
           cache: pnpm
       - run: pnpm install --frozen-lockfile
+      - run: pnpm --filter @datum-cloud/datum-ui build
       - run: pnpm --filter @repo/storybook build

apps/storybook/package.json

@@ -6,6 +6,7 @@
   "scripts": {
     "predev": "[ -f '../../packages/datum-ui/dist/styles/root.css' ] || ([ -d '../../packages/datum-ui/dist' ] && node ../../packages/datum-ui/scripts/copy-styles.mjs || pnpm --filter @datum-cloud/datum-ui build)",
     "dev": "storybook dev -p 6006",
+    "prebuild": "[ -f '../../packages/datum-ui/dist/styles/root.css' ] || ([ -d '../../packages/datum-ui/dist' ] && node ../../packages/datum-ui/scripts/copy-styles.mjs || pnpm --filter @datum-cloud/datum-ui build)",
     "build": "storybook build --docs",
     "lint": "eslint .",
     "typecheck": "tsc --noEmit",