Refactor to multi-category catalog with polished light mode

- Restructure from single filament table to multi-category product catalog
  (filamenti, stampaci, ploce, mlaznice, delovi, oprema)
- Add shared layout components (SiteHeader, SiteFooter, CategoryNav, Breadcrumb)
- Add reusable UI primitives (Badge, Button, Card, Modal, PriceDisplay, EmptyState)
- Add catalog components (CatalogPage, ProductTable, ProductGrid, FilamentCard, ProductCard)
- Add admin dashboard with sidebar navigation and category management
- Add product API endpoints and database migrations
- Add SEO pages (politika-privatnosti, uslovi-koriscenja, robots.txt, sitemap.xml)
- Fix light mode: gradient text contrast, category nav accessibility,
  surface tokens, card shadows, CTA section theming

public/robots.txt

@@ -0,0 +1,8 @@
+User-agent: *
+Allow: /
+Disallow: /upadaj
+Disallow: /upadaj/
+Disallow: /dashboard
+Disallow: /dashboard/
+
+Sitemap: https://filamenteka.rs/sitemap.xml