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>

lib/utils.ts

@@ -0,0 +1,106 @@
+/**
+ * Utility functions for the application
+ */
+
+import { type ClassValue, clsx } from 'clsx';
+import { twMerge } from 'tailwind-merge';
+
+/**
+ * Merge Tailwind CSS classes with clsx
+ */
+export function cn(...inputs: ClassValue[]) {
+  return twMerge(clsx(inputs));
+}
+
+/**
+ * Format a number for display with proper precision
+ */
+export function formatNumber(
+  value: number,
+  options: {
+    maxDecimals?: number;
+    minDecimals?: number;
+    notation?: 'standard' | 'scientific' | 'engineering' | 'compact';
+  } = {}
+): string {
+  const {
+    maxDecimals = 6,
+    minDecimals = 0,
+    notation = 'standard',
+  } = options;
+
+  // Handle edge cases
+  if (!isFinite(value)) return value.toString();
+  if (value === 0) return '0';
+
+  // Use scientific notation for very large or very small numbers
+  const absValue = Math.abs(value);
+  const useScientific =
+    notation === 'scientific' ||
+    (notation === 'standard' && (absValue >= 1e10 || absValue < 1e-6));
+
+  if (useScientific) {
+    return value.toExponential(maxDecimals);
+  }
+
+  // Format with appropriate decimal places
+  const formatted = new Intl.NumberFormat('en-US', {
+    minimumFractionDigits: minDecimals,
+    maximumFractionDigits: maxDecimals,
+    notation: notation === 'compact' ? 'compact' : 'standard',
+  }).format(value);
+
+  return formatted;
+}
+
+/**
+ * Debounce function for input handling
+ */
+export function debounce<T extends (...args: any[]) => any>(
+  func: T,
+  wait: number
+): (...args: Parameters<T>) => void {
+  let timeout: NodeJS.Timeout | null = null;
+
+  return function executedFunction(...args: Parameters<T>) {
+    const later = () => {
+      timeout = null;
+      func(...args);
+    };
+
+    if (timeout) clearTimeout(timeout);
+    timeout = setTimeout(later, wait);
+  };
+}
+
+/**
+ * Parse a number input string
+ */
+export function parseNumberInput(input: string): number | null {
+  if (!input || input.trim() === '') return null;
+
+  // Remove spaces and replace comma with dot
+  const cleaned = input.replace(/\s/g, '').replace(',', '.');
+
+  const parsed = parseFloat(cleaned);
+
+  return isNaN(parsed) ? null : parsed;
+}
+
+/**
+ * Get relative time from timestamp
+ */
+export function getRelativeTime(timestamp: number): string {
+  const now = Date.now();
+  const diff = now - timestamp;
+
+  const seconds = Math.floor(diff / 1000);
+  const minutes = Math.floor(seconds / 60);
+  const hours = Math.floor(minutes / 60);
+  const days = Math.floor(hours / 24);
+
+  if (days > 0) return `${days}d ago`;
+  if (hours > 0) return `${hours}h ago`;
+  if (minutes > 0) return `${minutes}m ago`;
+  return 'just now';
+}