feat: Fix product tables and responsive fonts

- Add product detail page ([slug].astro) with table rendering
- Display productTables from site-config.ts on product pages
- Add responsive font scaling for large screens (1280px+)
- Base font scales from 16px to 24px on 4K displays
- All text elements use responsive sizing (md/lg/xl breakpoints)
- Tables styled with green headers and alternating rows
- Add comprehensive documentation (FIXES_SUMMARY.md)

Fixes:
- Product specification tables now visible on product pages
- Font too small on large screens - now responsive

src/data/AGENTS.md

@@ -0,0 +1,94 @@
+# DATA LAYER - Product Catalog & Site Configuration
+
+**Generated:** 2026-03-01
+
+## OVERVIEW
+
+Centralized data layer for product catalog (~150KB total). Contains all product information, specifications, pricing tables, and site configuration in TypeScript.
+
+## FILES
+
+| File | Size | Purpose |
+|------|------|---------|
+| `site-config.ts` | ~149KB | Products, navigation, portfolio, company info |
+| `product-tables.ts` | ~33KB | Specification tables for products |
+
+## STRUCTURE
+
+### site-config.ts
+
+```typescript
+// Core exports
+export const siteConfig: SiteConfig        // Company info, contact
+export const workHours: WorkHours[]        // Business hours
+export const productCategories: ProductCategory[]  // All products (~20+)
+export const mainNavigation: NavItem[]     // Header nav
+export const portfolioProjects: PortfolioProject[] // Portfolio items
+```
+
+### ProductCategory Interface
+
+```typescript
+interface ProductCategory {
+  id: string;              // kebab-case ID (e.g., 'ppr-elephant')
+  name: string;            // Thai name
+  nameEn: string;          // English name
+  slug: string;            // URL slug (Thai)
+  href: string;            // Full path with trailing slash
+  image: string;           // Image path from /public
+  description: string;     // Full description (Thai)
+  shortDescription?: string;
+  keywords?: string[];     // SEO keywords
+  seoContent?: string;     // Long-form SEO content
+  specifications?: ProductSpecification[];
+  features?: string[];
+  applications?: string[];
+  certifications?: string[];
+  faq?: FAQItem[];
+  schemaData?: {...};      // Schema.org structured data
+  relatedProductIds?: string[];
+  productTables?: ProductTable[];  // From product-tables.ts
+}
+```
+
+## PRODUCT CATEGORIES
+
+| ID | Thai Name | Type |
+|----|-----------|------|
+| `ppr-elephant` | ท่อพีพีอาร์ตราช้าง | PPR Pipe |
+| `thai-ppr` | ท่อ PPR Thai PPR | PPR Pipe |
+| `poloplast` | ท่อ PP-R/PP-RCT POLOPLAST | Premium PPR |
+| `ppr-welder` | เครื่องเชื่อมท่อพีพีอาร์ | Equipment |
+| `hdpe` | ท่อ HDPE | HDPE Pipe |
+| `hdpe-welder` | เครื่องเชื่อม HDPE | Equipment |
+| `upvc` | ท่อ uPVC | uPVC Pipe |
+| `pvc` | ท่อและข้อต่อ PVC | PVC Pipe |
+| `syler` | ท่อไซเลอร์ | Fire protection |
+| `xylent` | ท่อระบายน้ำ 3 ชั้น ไซเลนท์ | Drainage |
+| ... | ... | ... |
+
+## CONVENTIONS
+
+**Adding a new product**:
+1. Add to `productCategories[]` array
+2. Create unique `id` (kebab-case)
+3. Add Thai `slug` for URL
+4. Include `seoContent` for SEO pages
+5. Link `productTables` if spec tables exist
+6. Add `relatedProductIds` for cross-selling
+
+**Product images**: Store in `/public/images/YYYY/MM/`
+
+**URLs**: Thai with trailing slash: `/ท่อพีพีอาร์ตราช้าง/`
+
+## ANTI-PATTERNS
+
+- **DO NOT** import entire file in client components - it's 149KB
+- **DO NOT** hardcode product IDs - use constants
+- **DO NOT** edit `product-tables.ts` manually without checking all references
+
+## NOTES
+
+- File is large but tree-shakeable for unused products
+- `portfolioProjects` also defined here
+- SEO keywords array should include both Thai and English terms