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

database/migrations/023_create_product_images.sql

@@ -0,0 +1,12 @@
+-- Migration: Multi-image support for products
+
+CREATE TABLE product_images (
+  id UUID PRIMARY KEY DEFAULT uuid_generate_v4(),
+  product_id UUID NOT NULL REFERENCES products(id) ON DELETE CASCADE,
+  url VARCHAR(500) NOT NULL,
+  alt_text VARCHAR(255),
+  sort_order INTEGER DEFAULT 0,
+  created_at TIMESTAMP WITH TIME ZONE DEFAULT CURRENT_TIMESTAMP
+);
+
+CREATE INDEX idx_product_images_product_id ON product_images(product_id);