Build inventory bay route with low-stock panel

[iamasx]

Summary

• refit inventory-bay into a standalone stock overview with band summaries, category tiles, and a compact low-stock panel
• replace the old recommendation-driven data flow with dedicated mock stock-band, category, item, and low-stock alert data
• update the route tests to cover the main render paths and responsive layout hooks

Testing

• npx -y node@20.19.5 node_modules/eslint/bin/eslint.js src/app/inventory-bay
• npx -y node@20.19.5 node_modules/vitest/vitest.mjs run src/app/inventory-bay/page.test.tsx
• npx -y node@20.19.5 node_modules/next/dist/bin/next build

Closes #171