feat: implement Phase 2 - Core conversion engine and UI

Complete Phase 2 implementation with working unit converter:

Core Conversion Engine (lib/units.ts):
- Type-safe wrapper for convert-units library
- Support for all 23 measures with TypeScript types
- getAllMeasures() - Get all available categories
- getUnitsForMeasure() - Get units for specific measure
- getUnitInfo() - Get detailed unit information
- convertUnit() - Convert between two units
- convertToAll() - Convert to all compatible units
- getCategoryColor() - Get Tailwind color class for measure
- formatMeasureName() - Format measure names for display
- searchUnits() - Fuzzy search across all units

Utility Functions (lib/utils.ts):
- cn() - Merge Tailwind classes with clsx and tailwind-merge
- formatNumber() - Smart number formatting with scientific notation
- debounce() - Debounce helper for inputs
- parseNumberInput() - Parse user input to number
- getRelativeTime() - Format timestamps

UI Components:
- Input - Styled input with focus states
- Button - 6 variants (default, destructive, outline, secondary, ghost, link)
- Card - Card container with header, title, description, content, footer

Main Converter Component (components/converter/MainConverter.tsx):
- Real-time conversion as user types
- Category selection with 23 color-coded buttons
- Input field with unit selector
- Grid display of all conversions in selected measure
- Color-coded result cards with category colors
- Responsive layout (1/2/3 column grid)

Homepage Updates:
- Integrated MainConverter component
- Clean header with gradient text
- Uses design system colors

Dependencies Added:
- clsx - Class name utilities
- tailwind-merge - Merge Tailwind classes intelligently

Features Working:
✓ Select from 23 measurement categories
✓ Real-time conversion to all compatible units
✓ Color-coded categories
✓ Formatted number display
✓ Responsive design

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

components/ui/card.tsx

@@ -0,0 +1,78 @@
+import * as React from 'react';
+import { cn } from '@/lib/utils';
+
+const Card = React.forwardRef<
+  HTMLDivElement,
+  React.HTMLAttributes<HTMLDivElement>
+>(({ className, ...props }, ref) => (
+  <div
+    ref={ref}
+    className={cn(
+      'rounded-lg border bg-card text-card-foreground shadow-sm',
+      className
+    )}
+    {...props}
+  />
+));
+Card.displayName = 'Card';
+
+const CardHeader = React.forwardRef<
+  HTMLDivElement,
+  React.HTMLAttributes<HTMLDivElement>
+>(({ className, ...props }, ref) => (
+  <div
+    ref={ref}
+    className={cn('flex flex-col space-y-1.5 p-6', className)}
+    {...props}
+  />
+));
+CardHeader.displayName = 'CardHeader';
+
+const CardTitle = React.forwardRef<
+  HTMLParagraphElement,
+  React.HTMLAttributes<HTMLHeadingElement>
+>(({ className, ...props }, ref) => (
+  <h3
+    ref={ref}
+    className={cn(
+      'text-2xl font-semibold leading-none tracking-tight',
+      className
+    )}
+    {...props}
+  />
+));
+CardTitle.displayName = 'CardTitle';
+
+const CardDescription = React.forwardRef<
+  HTMLParagraphElement,
+  React.HTMLAttributes<HTMLParagraphElement>
+>(({ className, ...props }, ref) => (
+  <p
+    ref={ref}
+    className={cn('text-sm text-muted-foreground', className)}
+    {...props}
+  />
+));
+CardDescription.displayName = 'CardDescription';
+
+const CardContent = React.forwardRef<
+  HTMLDivElement,
+  React.HTMLAttributes<HTMLDivElement>
+>(({ className, ...props }, ref) => (
+  <div ref={ref} className={cn('p-6 pt-0', className)} {...props} />
+));
+CardContent.displayName = 'CardContent';
+
+const CardFooter = React.forwardRef<
+  HTMLDivElement,
+  React.HTMLAttributes<HTMLDivElement>
+>(({ className, ...props }, ref) => (
+  <div
+    ref={ref}
+    className={cn('flex items-center p-6 pt-0', className)}
+    {...props}
+  />
+));
+CardFooter.displayName = 'CardFooter';
+
+export { Card, CardHeader, CardFooter, CardTitle, CardDescription, CardContent };