Add search functionality

Implement a search interface that allows users to find content by keywords, tags, or title. Results should include all categories.

Completed

• ✅ SearchBar component with modal overlay (⌘K / Ctrl+K shortcut)
• ✅ Debounced API search (250ms) via /api/search route
• ✅ Searches title, summary, tags, AND full body text
• ✅ Keyboard navigation (↑↓ arrows, Enter to select, ESC to close)
• ✅ Click outside to dismiss
• ✅ Category icons in results (📅📚🔬📋)
• ✅ Loading spinner while fetching
• ✅ Search button with ⌘K hint in nav bar
• ✅ Zero new dependencies — built with React + Next.js built-ins

Implementation

• src/components/SearchBar.tsx — Client component (modal, input, results)
• src/app/api/search/route.ts — API endpoint returning JSON
• src/lib/content.ts — Extended searchContent() with body text search
• src/app/layout.tsx — SearchBar added to nav